Acquihires do often have employee upside, in the form of large stock-based compensation grants or bonus payouts, typically spread over a vesting period of several years. There's no point in the acquihire if you don't retain the employees, and relative to the cost to acquire the company these payouts aren't huge.
You're right that often people do have to interview to remain employed (and thus to receive these benefits).
You're right that often people do have to interview to remain employed (and thus to receive these benefits).