Because you are using the data on a different device that you prefer. You can make all the arguments you want, but you can't get around the fact that you *want* to use the data on other devices and not on the phone. The whole point is that if you aren't using that data on the phone, you are not jamming up the network (supposedly, assuming you abide by your contract). You want the option to jam up the network no matter what device you use, and that's not cool. Usage patterns are built into the data assumptions that carriers make - assumptions that go into building their network capacity, as well as determining their prices. Carriers assume that the phone is not your main movie watching device. It can be, but it usually isn't - even in your case because you want to watch it on a bigger, better screen. If you are using the data no matter where you're watching it, then you are hogging the network with a device not authorized to be on the network.
Once again, IF you think you would use all the data on the phone anyway, then by all means, use it on the phone.
In the UK, there don't seem to be any unlimited data plans at all. On a limited data plan, I could possibly see the case for free tethering, but not on unlimited plans.
So you do have some sort of metering. Unlimited, unthrottled plans don't (hence they are unlimited and unthrottled). Even your provider does bandwidth management. Your beef seems to be with the way that some American providers do the same thing.
