If you want the phone to come with a dialer (I think most people do), then some dialer will inevitably be forced on people. And unless you think phone calls are an optional feature, it makes perfect sense that the dialer cannot be deleted.
The only complaint I can see here is that phone manufacturers can make bad choices for their default, undeleteable dialers. Well, yes, just like they can make bad choices for other software on the phone. The only reasonable remedy for that is to buy a phone from a manufacturer that makes software choices you like.
That phone manufacturers can make bad software choices is not an argument against having replaceable dialers. Quite the opposite. A manufacturer can make the exact same bad dialer choice if the dialer isn't replaceable. The only difference is that if the dialer is replaceable you might be able to do something about their bad choice some of the time.
I don't see the connection here. Why shouldn't people be able to delete the dialer if they don't like the phone company's choice? I understand preinstalling one, but preventing people from choosing another if they want seems unnecessary.