> you can't take a signing bonus then not show up for work.

You can if your union goes on strike between you getting your signing bonus and your start date. Striking is not the same as quitting in the eyes of the law.

Striking is not the same as quitting. If negotiations were ratified next week, workers would return and could not face retaliation in benefits or pay for striking. The university could withhold pay for missed shifts during the strike and potentially clawback a prorated portion of the remission at best, but clawing back remissions in their entirety and charging late fees for not paying by a deadline is bad faith and won't be looked upon fondly by the NRLB. While the decision will likely end up being made by a court, to me this definitely seems like illegal retaliation and not a case of students biting off more than they could chew.

If you are part of a strike, then yes, actually, you are legally protected against some forms of retaliation, including possibly this form. We'll see how it plays out in court, but it is by no means as simple as you're laying out. This is not "people decided to not show up for work," it's a collective bargaining action against a very clearly hostile employer.