Most games have extremely tight margins. Taking away 30% of their revenue means that many of them fails and will never make another game or that their budget for the next game becomes significantly lower. Both of those hurts the consumers interests.

If it was just 5% or so then sure it wouldn't be very noticeable, but just taking 30% outright is huge and will push many from being viable businesses to being forced to stop. The appstore isn't like steam which gets you a huge ecosystem of extra features for your games and tons of marketing. I'd pay extra to be able to add a game to steam even if I bought it elsewhere simply because steam adds so many other things.