They block applications that interfere with the operation of other applications.

This would include an ad blocker that blocks ads in, say, a free game, or the built in Chrome browser.

The common workaround was to run connections through a VPN and your ad blocking happens there. I'm not sure if they started cracking down on that as well.

Having apps that modify other apps can certainly be desired behavior, but it's clear why there would be legitimate concerns about customer security in allowing apps that can do so.