It's pretty obvious they were holding up the bonus payments to retain their employees. And when that didn't work, they started to pay out the bonuses to keep who was left there. Whether it's legal or not, it's unethical and I for one won't be purchasing the next MW iteration from activation. It probably won't be much of a game regardless.
By the way, I was paid a nice 8k bonus even though I was no longer part of the company I had worked for, because I did the work required and earned it. Could the company have withheld it and been legally correct to do so? Absolutely. However in this case, it's called doing the right thing.