Federal law also says you can't sell marijuana. However the government has pretty much decided to not prosecute that for some time. In general it's hard to believe that any law such as that would not defacto allow an exception for what can be shown as an abusive behavior. Under the theory that the law says 'you are required to connect all calls' you could say that carriers could not legally block someone who keeps calling a phone number in an abusive manner (with or w/o the permission or at the request of the customer).
The big companies aren't going to move until they get some guidance on where the line between blocking some numbers and systematically blocking numbers deemed to be 'abusive' based on the companies own criteria. Of course today you can request specific number be blocked from calling you of course but no one is currently trying that because the dangers of crossing the blurry line can be severe.
Your comparison isn't much more useful than 'prosecutorial discretion exists therefore do any crime you want.' Multibillion dollar companies aren't going to just go out and blaze a trail like that. We even see this in the (spurious) comparison to marijuana, the first companies to step out and test the state vs federal divide were small and local and only after those companies figured out some of the issues did we get large and larger companies coming in. (Though they're still all independent arms because of the difficulties accessing banking because of the federal laws still in place!)
