Because Go itself won't let you transfer Mew to anywhere. You can't even get rid of it in your own game, you're stuck with it. The same goes to Celebi. Because there is only one of each per account, they're essentially tied to your game.
This was debated almost a year ago, and if people decided to check their facts before getting angry, we wouldn't be having this debate.
Pretty much, yes.
There is no precedent to blocking Mew specifically because they wanted to push the peripheral, because that's not how Go works. Refer to the first part of my post.
Obviously it made the cut, because it needs to exist in the Galar Dex to be in the game at all.