Your "edumacated guess" contradicts my direct observation.
The "enforceable" agreements are usually structured as a "loan", in other words, as a promissory note.
Otherwise, you start getting into "indentured servitude" issues. What if, for example, a company promises you that you will fly 90 hours a month and be paid $20./hr, or $1800./month. Then, the flying dries up somewhat, and now you are only able to fly 65 hours a month, or $1300. Are you still obligated to stick around, even though you have budgeted your family around the $1800. you were promised? What if they cut your flying back to 20 hours a month, and now you are making $400./month. Are you still obligated to remian there for a year? Well, where do you draw the line?
PArt 135 piston training is not that expensive. It is not like sending a guy to FSI for 3 weeks to get a G-V type. An employer has to make a decision . . . either treat pilots well enough that they stick around for a while, or pay for more training events . . . . it's not rocket science, and most companies manage to do it without a training contract.
One of the problems with this industry is the number of operators trying to do things on a shoestring budget, and another is pilots who are willing to work for next to nothing . . . . .