In California, for example, you can't even dock someone's final paycheck if they fail to return a uniform - if someone worked the time to earn a wage and you don't pay them in full, you're going to find yourself in court.
It's kind of mental accounting for the employee, though. The law firm is basically giving you an extra $100 or whatever per paycheck. However, this $100 is earmarked to pay off your law school debt, so in effect the company is helping to pay for your training.
The nice part for the company is that they do not take on any risk paying for the training: the employee already shouldered all the risk for paying for law school. The company buys a little bit of loyalty by helping the employee pay off the debt over time. It probably makes the employee think twice before quitting or looking for other employment; it is similar to the health insurance benefit in that respect.
Then it's not "clawing back" pay, it's just a fine for breach of contract.
I do think laws should prevent low-wage employers from screwing over their employees for piddling stuff like uniforms, but I think that's different than something like a professional apprenticeship.