Apple is doing absolutely nothing to prevent you from buying any charger that you would like. Buy a Belkin, buy a whatever. As long as it puts out actual 5VDC and enough amps to activate the charging circuit, you'll be perfectly fine.
But if you buy some cheap piece of shit that spikes voltage and blows out your phone, Apple is going to say that you used a cheap piece of shit that voids your warranty, just like any other device manufacturer would, regardless of if it's a phone or not.
Oh, but it's Apple, so clearly they are monopolistic shitheads all of a sudden, even if this has been the case for literally decades of portable electronics.
Worse - they are being advertised as "genuine" Apple chargers