I bet everyone would be up in arms if internet service providers made you pay a separate $20 charge for each device that connects to the internet through their network. That's just completely absurd, why should I pay a separate charge to connect each internet capable device to the internet. Off the top of my head, I can count 14 different devices in my house that I connect to the internet not including the three smartphones we have.
But that's because the terms it was sold, nor do they have the same capacity constraints. VZW never sold their service as a broadband replacement, so I don't see the issue. Now, if they want to cap then I agree data is data, but selling unlimited data on one device vs. multiple devices is clearly different and the usage would be significantly different. When people violate that TOS, data usage increases and it changes the dynamic (i.e. the average person uses 2gigs on their data plan, but with tethering replacing broadband it could be 10-20 gigs) forcing VZW to either cap or hammer down on tethering.
And, by the way, some broadband providers are talking about capping data (150-200 gigs). Between torrents and people replacing cable with Hulu and Netflix and the like it's changing the dynamic, especially when you consider the cable provider is providing both cable and broadband. If people are trying to game the system replacing cable with broadband it shouldn't be surprising that they will make-up that loss by charging high bandwidth users more.