Impending sale is a unique situation where some of the other motivators are in serious doubt (control, status, etc.). No one can be credibly offered the other motivators when there's a chance of complete ownership change in the near future. ("The new owner might promote you; they might fire you. We can't guarantee either outcome because what we're selling is the option value.")

None of the studies of motivation I've seen have specifically addressed such a situation. They concern motivating good work, not sticking around through a foreseen ownership discontinuity.

A stay package may not make people happy and productive, but it does contractually keep them around, giving the new owners a reasonable chance of receiving the knowhow and functioning business, before eventually making their own personnel/motivation decisions.

(That's why even though Fred Wilson is right about the stay packages' propriety and value, I don't think there's anything wrong with calling them 'bribes'. That's a vivid analogy for the sort of purchased-but-not-necessarily-enthusiastic compliance they can achieve.)