Why doesn't Canada do what you ask? Why isn't it Canadian Forces in there defending the Kurds, even if it means Canada going to war with both Syria and Turkey? Why must it always be the United States? (While our ostensible "allies" insult us behind our backs and pretend to be superior and less militarist?)
Frankly I wish Canada spent what it's supposed to be spending on national defense, and even more importantly I wish it were spent in an efficient way that achieved results; our annual military budget is already higher than Israel's, but I don't think anyone would think to give us the advantage if we engaged them anywhere outside Canada. But to our credit, we never told the Kurds that we'd protect them from everyone else if they support our fight against ISIS, whereas my understanding is the US did just that.
The Kurds will only reject an alliance with the US as you suggest, if they think that they have a better alternative. That's not looking very promising for them right now.
You miss the point, it's not specifically just about the Kurds. Next time the US finds itself engaged in some sort of dispute or conflict and the smaller players find themselves having to choose a side, they will look at how the US treated the Kurds in their time of need and decide whether to cast their lot in with the US on that basis. If you declare yourself someone's ally, and accepting that alliance automatically makes your allies into someone else's enemy, you need to stick with those allies when their enemies come to punish them for taking your side. Unless the US plans on committing genocide in the Middle East and replacing the population with American settlers, you will need local allies to get anything done in the region.
Many of the other Arab countries are very concerned about the rise of Iran. Iran is very close to having nuclear weapons and it already has ballistic missiles capable of hitting its neighbors. They are developing the ability to close off the Persian Gulf to shipping and hence dry up petroleum exports to Europe and East Asia. With the prospect of Iran soon becoming a Shi'ite Islamic superpower, its neighbors feel a need to ally themselves with a strong military power capable of standing up to a rising Iran. That's what motivates them, it's their interest.
They are going to have that interest regardless of what the US does. It's kind of inherent in their situation. The only question is whether the US is the best place to look for a protective alliance. China hasn't stepped into that role yet. Russia would like to, but with Syria already on their plate, they don't really have the means. The European Union has the economic and industrial clout to do it, but they have avoided developing their own military capabilities like the plague. They have always felt that it was their interest to off-shore their own military defense to the United States military (at US expense) while devoting their own funds to their own welfare-states.
I'm fully in agreement with your position on Europe's failure to fund its militaries to implement the global policies they call for, and that's one reason the US isn't required to take their interests into account when deploying its own military to solve problems, just like it doesn't need to account for Canada's either. But you neglect here to mention America's
own interest in the matter, which would be to prevent its trade partners in the Middle East from making a deal with Iran that amounts to surrender and blocks the US from doing business in the region, also to prevent Iran (with Russian and Chinese assistance) from dominating the region and aligning it against you, which are both clear Iranian goals.
Israel doesn't need any help because it has enough nukes and other WMD's to trigger global armageddon in case no one else wants to defend it, or they could simply start annexing territory from anyone who threatens them while whiny foreign hypocrites complain, but if Americans don't come through on their commitments to allies in their times of need, Arab countries will either be allying with hostile powers, or building and purchasing their own nuclear arsenals just like Israel. If I were Ukrainian, I'd already see it as a critical historical blunder to dismantle their post-Soviet nuclear arsenal, based on the expectation that the US and Europe would go to war with Russia to protect them or at least enact crippling sanctions. So imagine what's going through the heads of every other US ally or would-be ally when they consider their options for self-defense, and ask yourself if it's in America's interests to see all these nations and militias freelancing.
So when it comes to the role of 'potential military ally' for weaker countries that feel threatened, the US is just about the only game in town.
As I say, they can always whore themselves out to Iran, Russia and China if no one else will help defend them, and those are countries so obsessed with harming the US that sometimes they even harm their own interests in order to do so.
The obvious question that arises then is what is in the United States' interest? Is it in the US interest to get into a ground war with Syria and Turkey? Probably not. The Kurds don't offer enough.
ISIS was at the bottom of Assad's priority list (except when preaching to his KKK friends in the US), which is why America got directly involved in the region to begin with. Indeed, Assad used those same terrorists to kill over one million Iraqi civilians and keep American troops bogged down in Iraq for a decade. The Kurds offer you plenty, and you could at least threaten to give Turkey the boot from NATO and a lot of other exclusive clubs if they intervene. But as I've pointed out, it's not only about the Kurds, it's about having allies stick their necks out for you when you need them to do so, without them fearing that you'll abandon them as soon as Henry Kissinger thinks it's convenient.