Enforceable...in Europe/Asia, generally yes.
In the USA, generally no.
Unfortunately, for those that do intend to remain with a company after training (and why not, after all, when you applied, you asked for work, not a vacation) have, in many cases been "tarred with the same brush" by those that went before, and ran away before completing their contract(s).
You must understand that airline companies offer rather expensive training on type and EXPECT you to stay.
Personally have little time for those that don't.
OTOH, companies should only consider bonding once, and once only. To repeat seems to me to be over the top.
IF companies insist on bonding a second time, management needs to look in the mirror and ask themselves..."what are we doing wrong"...and of course most will not.