Well, there'd be rioting in the streets if it turned out that typical cell phones were constantly reporting GPS data to the carrier, especially if they still did it when the GPS feature was supposedly turned off. Why should a MiFi be any different?
There is a big difference between a device that in effect acts as a cell site (broadcasting) versus a subscriber handset.
The carrier still has that data, though, and they could be forced to turn it over when faced with a subpoena or warrant -- or, as we've seen repeatedly, they could just decide to turn it over in response to a polite request from the government.
This has nothing to do with your argument. I am speaking from an engineering point of view. However, if/when instances of this occur, people need to sue the Fed (under violation of the Constitution) and the carrier (secondary).
If your main worry is what a large corporation will provide the Fed (warrentless or not), stop using their services (i.e. a mobile) immediately. at&t is one of the largest, if not the largest lobbyist in the Washington. The others are not far behind. Stop screaming bloody murder about privacy, and actually DO something about it. Maybe go HAM. Personally, I don't care. I have close friends who are Afghan and Iranian, and I talk with them on my mobile all the time; I assume my shit is being tracked.