Oh I know that trust me I do but we dont in normal legal system punish people for the actions of others (not with standing conspiracy which is bloody hard to prove)

This system lends it's self to blackmail "hey ex boss give me 50% of that hold back money you got from the acquisition or I will leave and f&*k you over".

ps Yes I am aware of the "joint enterprise" issue in UK employment law where you can legally fire all of a group of employees if you can prove that one of them did something realy naughty.

Ok so stated a different way as follows.

"I will pay you $1,000,000 for your restaurant. If the chef in the kitchen is still working for me after 1 year I will pay you an additional $50,000."

I would say that thats the chefs 50,000 :-) pay the retention bonus to the ones you want to keep not the previous employer.

It doesn't preclude a similar retention bonus for the chef. The point is, the leadership of the selling firm has some influence over whether their employees stay or go. The buying firm sets up a financial incentive to encourage the selling leadership to use that influence.