That's a choice the apps make about how to structure their model. They do have the same access to the reward pool as steemit.com, they just choose not to make use of that.

Anyway, if you mean that these apps should have an opportunity to be subsidized by stakeholders without getting involved with posts, comments, votes, etc., then SPS (expected to be part of the next hard fork) is what you are looking for. App developers will be able to make a proposal on how their app adds value and stakeholders can vote to provide funding for it.