Which, in fact, they can-- if you use their products.
There is, however, a straightforward way to stop them: Don't use their products.
If others aren’t happy with it that’s fine too, just get a different phone. I don’t see any justification for coercing Apple into entering into commercial arrangements and developing product features they don’t want to support.
I‘ll admit that having the Apple iOS ecosystem is of certain value, but Apple doesn‘t even allow you to install your own OS. My point is that if I buy a hammer, should I be forced to use it only for hammering nails or could I use it for other things as well?
The reality is, you bought a product that accurately advertised its features. Were you ever misled into thinking that an iPhone could download non-AppStore apps? If so, please return your phone you are owed a refund and maybe even sue for false advertising.
Meanwhile, if you bought the product like me knowing the product’s features. It feels disingenuous to now want more from the product.
If you want to jailbreak your iPhone, do it. But you really shouldn’t expect Apple to provide updates or support your warranty once you choose to. The same as if you opened up a hair dryer, and disabled the part that heats the air the warranty would be void.