Don't forget, the way that DSL technology works is that it's a direct line. There is no sharing outside of the central office, so there is no real stress on the lines. If there was, all DSL users would have constant issues with there lines.
AT&T is doing this only to make more profits on people they already have.
Thing is, they could have simply advertised that they offer unlimited bandwidth and most likely got more customers and made a lot more money that way.