an acquisition does not normally require existing employees to sign new contracts
I'm not sure why you think that is true.
So this is a non-negotiable reduction in the legal position of the employees at best.
This is an extraordinary statement that requires the requisite evidence. I don't think it matches US law or precedent, it simply seeks to formalize it, while giving the other party a full and fair negotiating position.
Every single case I can think of where employer and employee/buyer got into such a spat are the ones where such agreements were not in place. Can you think of a counterexample?