Both muting and blocking on client side do not restrict the sender in any way. It is a fine way to prevent unwanted messages.
On the server or forced upon the sender are a lot more problematic, because you take away choice from receivers who might want to receive the message.
This is like saying the USPS temporary ban on delivering Chinese packages didn't restrict the sender because they were still able to box and mail the package.
If a recipient doesn't want messages from a sender that's fine and if they want to opt in to a 3rd party adjudicators WOT/ spam etc filter that's fine too (I expect most people would opt in to this too). The issue is where it is opt out and particularly that it's not transparent to new users that the function is enabled by default.
