I'm confused now. I thought customers that already had the unlimited iPad subscription would be allowed to keep it
They can, as long as they continue to re-subscribe to the $30 plan (which happens automatically unless you cancel). Once you cancel, though, you're stuck with the new plans.
To some extent it's still a bait and switch - for example, people (me) bought the ipad 3g with the promise that we could activate the $30 when we needed it, and deactivate it when we don't need it. That is no longer true - to have true unlimited, you need to continuously pay $30 each month, even if some months you plan on using 0 minutes.
There has always been a cap, it just AT&T never really did anything about it. Now this makes it easier to enforce.
as often as that chestnut is repeated, no one seems to be able to cite any evidence for it, and at&t seems to have continuously denied it.