Well, AT&T for example requires that you use their provided modem+router combo, which they provide for free (unless you include their plans being generally more expensive than their competitors as an extra fee). They do try to sell you on range extenders for, what I assume to be, the shit router they give you.

Their router gives you less control than you'd get with your own router, helps with lock-in because it makes it harder to change providers, and allows AT&T full root access to your network, so I wouldn't recommend it for self-hosters. However, it is the cheapest option since you're requited to use it anyway. Besides, of course, using a different ISP, which saves me tooons of money over AT&T.