I think you’d be surprised just how many Amazon fire tablets are out there. They don’t get much mindshare because they’re cheap but I run into a non-trivial number of them at friends/acquaintances houses. Especially for kids.

Hard to know since Amazon doesn’t release sales numbers.

Either way Apple isn’t doing anything to shut down tablet competitors other than making a great product at a price people seem to be willing to pay. There aren’t tons of lawsuits or requirements that retailers can’t sell competing devices.