How is forcing a data plan legal?

[martypng]

I am interested in understanding how it is legal for providers to force a data plan. If they are subsidizing a phone under contract I understand they need to recoup the cost, but what if I bring my own phone and don't want a data plan? How is that in any way legal? That would be like if we were forced to pay the tolls for all toll roads even if we never drive on them.