Throttled

While I suspect this might have happened to me (I have a grandfathered AT&T unlimited data plan that sometimes is mysteriously slow), what I don't get is why AT&T would do it. If I get too frustrated with AT&T's service, I could switch to another carrier. It isn't going to get me to give AT&T more money, especially if they are not telling their unlimited users about the throttling policy (and offering them a deal where they could pay more to avoid it).