If you don't show with "good" reason (finances, health issue, etc) they usually do nothing.

If you don't show with a "bad" reason then what they do varies, but I've never heard of someone being sued. Instead it seems to be things like, "we will tell your high school and it may be mentioned if you try to apply to another university."

It's documented at https://www.nacacnet.org/globalassets/documents/advocacy-and... as "if a candidate has been admitted Early Decision, then the college may notify the candidate’s high school and other colleges of the candidate’s application status." This is linked from https://www.nacacnet.org/advocacy--ethics/SPGP-NACAC-Code-of... on the NACAC's website.

Often the college will blacklist the high school the student went to and nobody will be able to get in early decision for the next few years