Because normally these mons have limited options and there's a ton more room for abuse now there's a sketch move. It may not be an issue in OU due to checks and game balance, but game balance is really wonky in sketchmons to the point of broken. Remember this isn't OU: ANYTHING can do baton pass and/or boost it's attack. This is why I said it needs banning on all mons in the sketch league as it's super abusable with no checks outside of itself. Having it on mons that can already learn it only frees up room for an extra sketch move to make it worse.Why all mons? Why did it suddenly become overpowered on Dragonite and Arcanine if regular OU can handle both just fine WITHOUT extra coverage.
... why take it away from mons that only mostly use it already?
If there was more than one move with +2 priority it wouldn't be an issue as then we could check it with other moves besides extreme speed. There's just zero checks, especially now there's countless ways to power the move up and/or increase speed.