They don't have a choice, generally speaking. Apple makes it harder to get around the fee. If they're successful on Android, it's possible they'd try to swing their weight around on iOS... but I suspect Apple doesn't care how big you are.
It’s much harder to side load apps on an iPhone.
We will not be pushed around! We will revolt, but only if it's easy!
Well.. that and average iOS users can't easily side load either. They need to jailbreak and that, itself, is a difficult task.
You don't have to - you can sign IPAs with a developer certificate, but the app will only work for 7 days on a free account (1 year for a 'real' developper account). Having to resign every week is obviously a dealbreaker.
This sounds like one of the Viva la Robolution slogans.
They aren't sideloading on Android either, so I don't see the relevance.