It appears that part of it is a bit of a technicality—Apple never allowed a market for app distribution on their platform at all, while Google did. So, technically, Google is abusing their competition while Apple simply doesn't have any. There's a certain kind of logic to that argument, but it feels like it establishes perverse incentives for a platform to retain 100% control. It's legal precedence for "give them an inch; they'll take a mile"—not only will they try to take the mile, they're legally entitled to.
I also suspect that the other side of it has to be that Google is simply far less competent at managing its legal position than Apple is. Google is constantly getting into trouble in these cases for deleting data they shouldn't have and similar offenses. Apple, meanwhile, doesn't seem to retain any more incriminating data than Google does, but they somehow simultaneously manage to avoid getting in trouble for their data retention policies.