They were banned due to didymo.
Ongoing studies are finding didymo spores in virtually every waterway in the northeast. It is apparently, native. We just didn't know it until blooms starting showing up in places we hadn't seen blooms.
But that's due to water chemistry changes, which, yes, are due to human activity. But have nothing to do with felt soles.
Now, how far south didymo is "native", and where it's "invasive", is still somewhat in question. Vermont, for sure, it's native. They tested a bunch of streams and found spores in ALL of them. Maryland? Nobody knows yet. But at this point it wouldn't at all be surprising to find out it's native to basically all of the U.S. and present in nearly all waterways.
Further, while yes, felt soles are capable of spreading it. So are birds which fly from place to place. And fish and frogs and raccoons. And even wind. Yes, just wind.
So regarding the felt sole ban. It was put in place to prevent the spread of didymo. Didymo is perhaps native, and even if not, it's already present in nearly every waterway. Hence, the felt sole ban isn't needed.
At least for this purpose. There are other invasives that may be spread by felt, but the fear factor on those hasn't escalated to the point where the public would demand a ban on felt.