Because if I own the device, manufacturer should not decide what kind of apps I can run.
See: a user in China owns the iPhone and wants to run the app.
Apps developer wants a user to run the app.
And only Apple doesn't let it happen, on the device that it doesn't own anymore.
Regarding your argument, "do not buy it", of course I don't buy it! But that doesn't make this position of Apple right, and I'm glad that lawmakers might put an end to this malicious practice.
Returning your argument, if Apple wants to control what users do with the iPhones, perhaps they shouldn't give up their ownership of devices and shouldn't sell them.