Apple has told several social-networking apps to disable their ‘tip’ functions
wsj.com
wsj.com
Paypal or some other large entity could probably do this, though.
In the realm of "should", I'm afraid regulation is your only bet.
Otherwise obviously any company will take whatever % they think will get them the maximum long term profit
There ARE dozens of app stores. These two are very very strong ones. The thing is how long Apple and Google and continue to squeeze people until someone will come up with a clever trick to bypass their payment system.