Plus they're outright wrong that it constitutes "fraud," if for no other reason than the person skipping additional legs didn't profit from it, instead they simply limited UA's ability to further profit off of their travel.
If this was "fraud" then any other airline discount or saving would be too. Shop around for the cheapest price? That's "fraud" since UA didn't make as much. Didn't check bags for $50+ and instead overload your carry on? That's "fraud" since UA lost the checked bag fee. Fly to a smaller airport instead of a larger one, then take the bus? That's "fraud" too, UA deserves that money.
UA's creative use of the law here is nothing more than an intimidation tactic. They've run out of ideas so instead are just trying to muddy the waters enough to stop this becoming overly popular. I guess that's easier than re-examining how you ticket/what your business model is.