They wouldn't know with certainty. But many people knew when they were messing with the fastpass pools back when FP+ was being implemented. Standby would start moving a lot slower, fastpasses would start running out a lot faster, there are anecdotal clues the people would dissect online.
A whole new program can't just be snuck out of the old inventory without people starting to talk and wonder what's up.
And illegal? Of course not. Their circus, their monkeys. But ultimately it comes down to guest satisfaction, which I believe is down in general right now. They can't afford to let it slip even more.
Satisfaction is down yet revenue is up as long as revenue grows the satisfaction metric is irrelevant to TDO