Granted, it sucks to see this happen, but if transfer rates aren't also specified in the plan/advertising it's hard to argue that this represents going back on an "unlimited data" promise.
I would say yes: if you have, say, a 1MB/s line, which you keep constantly saturated, you can transfer (1 month * 1MB/s) = ~2.5TB in that month. Whether you have a data cap or throttling, it translates to such a line getting being able to transfer less than 2.5TB of data per month.
You'd say "that's absurd; it's not high speed internet. You've effectively limited my speed." They advertise _unlimited_ and _high speed_. If they throttle, then one may have a case depending on the reasonable expectations one might have about how fast (total throughput) a service labeled as "high speed" should be.
(IANAL - this is not legal advice)