Well, as a provider, they are not legally allowed to ban customer owned eqipment. By law they are required to let customers use their own equipment. But all the services are still controlled by the provider.
Unfortunately, it's not a violation of DMCA since as a customer, you are allowed to suspend your DVR service. That doesn't require a different box, the provider just eliminates your access to DVR functionality. If that doesn't violate DMCA, why would blocking the ability to FFW through commercials? Sorry to say, it doesn't.
You also should try to keep in mind that a lot of your service functionality is not decided on by the service provider, but instead contracts with the content providers (NBC, CBS, Viacom, etc).
Which is why Dish network put themselves in hot water with the network companies over their Hopper dvr box, which allows customers to skip commercials on recorded programs all together.
That doesn't directly apply in this specific case, but it could also be what is pressuring TWC to do this. Either way, it's not a violation of DMCA.
Source: I work for a major service provider. We suck, I know, but unfortunately everything I said is true.