You have to remember that they aren't preaching to Catholics here.
Their target audience are the millions of the religious far right in America who are predominately not Catholic and who believe the Church is led by a Liberal who criticised Trump for his border policy, not to mention guns, their policies against the poor, Muslim countries, and how the Pope has directed his ire at EU and US governments for their policies against refugees.
The one thing they had in common with the far right Christian groups in the US has been their stance on abortion. So when they act outside of the purview of the Vatican and attempt to ban the Biden's from receiving communion because of their stance on abortion, they aren't preaching to Catholics but to the Christian right in the US. After all, the far right Christians predominately do not like Biden. What better way to try to peel off new members than to deny the person they hate the right to receive communion on an issue both sides vehemently agree on?
The Bishop's attempts to deny Biden communion has nothing to do with religious dogma and everything to do with politics.
Catholics still make up the biggest chunk and while their numbers are dwindling, with the far right increasing, this is all about politics.