The argument essentially is that it hints to there being an exploitable slack in the system. If you treat bargaining with employees as a zero-sum game (which i

The argument essentially is that it hints to there being an exploitable slack in the system. If you treat bargaining with employees as a zero-sum game (which it really isn't, but some companies do treat it that way) then the other party being very satisfied with the bargain indicates that they would be also willing to get a worse offer. So if all the employees are extremely satisfied with the compensation/effort ratio, that indicates that you could either lower the compensation some or extract more effort until they're satisfied "just barely enough".