Canada, not the U.S. here, but I've worked for two companies that had training bonds.
In both cases, the companies were pretty lax about letting pilots off the hook for training bonds until one day they weren't. Once they decided to go after the money, it was madness. One of the companies chased a pilot for $500 out of principle. It ultimately cost the company and him far more than $500, but it set the tone for everyone else. I know other people who were offered an opportunity they could not afford to pass up and came to terms with the company for a payback scheme.
Ultimately, contracts have a cost to breaking them. If the company breaks it before all the terms are met (outside of dismissal for cause), they owe you severance. If you break it before all the terms are met, you owe them. If you don't like the terms at the beginning or don't think you can work through it, don't sign. It is really that simple.