Not sure why you wouldn't. Modem rental costs, what? 5 or 10 bucks a month? Just buy a modem and it pays for itself pretty quickly. I'm honestly surprised that they let you do it.
Otherwise what you mean is "it is mandatory to lease their router even if you have your own, and you are forced to enter a 1-year minimum contract.".
Yes. you also have the option to just pay for the ISP router outright.