Here in New Hampshire, an insurance company was going to drop a young college student with cancer because her coverage was through her parents' policy as a full-time student, and the cancer treatments forced her to cut back her course load and lose her full-time student status.
This resulted in the passage of HR.2851, signed by President Bush just last year. So it's imprecise at best to say that it's been illegal for "over a decade."
Adults (which now includes kids over 18) are not allowed to be on a "family" policy because they are not minor children.
The policy makes an exception for full time students still dependent on their parents. If somebody ceases to be a full time student residing at home, they fall outside the exception and are not covered. Usually they drop out, move out, or go to work. A dependent student being unable to continue her studies because of illness is an anomaly.
So it has no relation at all to cancelling an actual policy holder for illness. And yes, THAT has been illegal for many years.