It's not either/or. If you have people like Merseyrail's enforcement officers, they can and should enforce against ALL antisocial behaviour. Nothing is gained by having them limited to a subset of it.
You're going to struggle to get the same level of enforcement on buses as you will on Merseyrail. You don't even get the no feet enforcement on buses in the Mersey area. Plus, to many (especially younger) feet on seats isn't anti social as it's not affecting anyone else like vaping/smoking is or loud music being played.