The specific case of charging for equipment not provided is one part of the law, but by no means the only one, or even the main focus. I quoted the relevant sections in another thread a couple of months ago.
As I recall it makes it illegal for a provider to make equipment rentals mandatory when a consumer is able to provide their own, for any "covered services" which included all broadcast and internet (not just cable companies). Charging for equipment not provided would be just one of the many sleazy ways of essentially making the rental mandatory.
On the surface this would apply to the AT&T router - internet is one of the covered services, and routers are standard equipment that anyone can by for themselves. But AT&T will no doubt argue that the law doesn't apply because there are technical reasons you can't use your own router: as discussed in another thread, their routers contain a certificate that is needed to authenticate to their network, so you can't just use any router. This is, of course, basically nonsense. There is no real technical reason for AT&T to require an extra authentication step to connect a router to a physical line that an AT&T tech ran from their infrastructure into my house. But it would probably take a class action suit, or at least an adverse ruling from a regulatory body, to get them to comply with the intent of the law. I am not holding my breath.
I should add that it also apparently doesn't apply to Sonic's analog telephone adapter (which was the subject of the other thread when I looked this up). Again, there is no technical reason that a consumer should not be able to provide their own equipment, but voice service, unlike internet, is not covered by this particular law.