there is no "rule" or standard really for this.... but in most cases apps have beneficiaries, meaning that when a user makes a post the app's main account is added to the post and receives a portion of the payout.
Some other apps use your upvote as form of payment, meaning that once a day sometimes or more they will trigger your account to upvote one of their posts.
This is to say, there is nothing wrong with a method of making sure devs and platforms built on steem are designed in a way where the team in charge of said product makes a decent living. It should however be transparent and clearly explained to all its users.
Thanks @meno for that explanation, I had been wondering how devs made money from Steemit apps, I actually tried last night to find info on this without success.
Given that 2 methods exist currently, all can disregard my alternative suggestion, given above.