Whoever told you it is legal. They just decided to not pick that battle.
Newhire bonuses have been added to other CBA's also since the Gojet settlement, maybe AWAC, or TSA I can't remember. The company knows there is exposure and just lives with it.

One of those double standards, wouldn't be able to cash the first check if they tried it at mainline.

Its textbook unauthorized "self help" by the company.