But let me ask you something. Do you think the rules that force telephone companies to connect to everyone are a bad thing? I think the way that's been handled is good, and that forcing data caps to apply equally is pretty similar.
Data caps still apply to all T-Mobile wireless subscribers also, although I can see that the spirit of your argument is that carriers shouldn't try to exert influence over data endpoints (typically referred to as "content"). But, using your same POTS analogy, does this mean that 1-800 toll-free numbers should have been banned or regulated out of existence? The interesting point is that in this case, T-Mobile is working around the FCC Net Neutrality laws by making it clear that this is a free program for specific content partners to participate in; it's like "free toll-free" for content that might appeal the most to consumers (streaming audio and video).
So there were very good reasons the government got to dictate their business models. It was quid pro quo.
But I don't think interchangeability is the same as treating them equally. AT&T had to connect you to anywhere globally, but it didn't have to do it for the same price. Local vs. long distance vs. international.
Some cellphone companies even had "pick 5 free number" plans. Nothing bad happened.
Plus the internet seems to achieve total interactivity without it being mandated. Though a lot of the backbone internet came from regulated telecomms. Maybe I can't count on the internet developing in a free for all.
That fits in with saying that broad categories are fine, but you can't charge extra for me to call specific stores.
The internet is doing pretty well, but you get ridiculous things like ISPs wanting Netflix to pay to peer even though all the traffic is initiated on the ISP end.