The rule and the fee are complete bullshit. 30% isn't even remotely viable. They should be charging equivalent to transaction fees, which would tally below 5%. Hopefully one day someone will find a law with which they can challenge the fee's insanity in court. Anti-competitiveness seems like the appropriate avenue; we just have to wait for a company to try their hand at it.
At a bare minimum, apps must be allowed to explain to users in-app that they can reduce their payments by paying outside the app.