Unclear until the pricing comes out, they may just include it for free when you buy internet service. But ISPs generally need to package at least something to serve Wifi for customers that don't already have their own Access Point, looks like SpaceX just put together their own, rather than provide something off the shelf.
Most ISPs don't require this -- you can use your own equipment if you wish. Most customers take them up on the rentals, though, either because they don't know any better or because it's easier than buying and maintaining their own equipment.
AT&T is the worst. Not only do they charge for a useless router, but there’s no way to disable or bypass it. It has a DMZ mode that supposedly lets you double NAT to keep their spyware crap off your LAN, but it doesn’t seem to work in practice.