Internet providers only have to allow this $50 discount toward plans of their choice. I have read that in many cases, to get the discount, the customer has to upgrade to a higher-cost plan. Then once the discount is no longer available, the customer is stuck on the higher-cost plan, especially in the case where the customer was grandfathered-in on a lower-cost plan that is no longer available.
The FCC should have just given consumers the $50/mo instead of making it a credit to providers. Internet providers are snakes, and if there is a way they can bite you, they will.