Because most people are too stupid to hold out until a fair contract is offered, and when they do some dumber fuck takes the previous offer and proves their hesitation correct. Sadly, most people in this country are far too stupid to be trusted to do the right thing on their own, so we make rules like this. Your rationalizing of why its okay to fuck someone out of their livelihood is lazy and lacking in real thought.
And BTW, most people affected by this ARE hourly workers (as a matter of fact, if you are not hourly, how do you calculate time and a half? Because if you aren't being paid by the hour, it's kinda hard to calculate)