From what I've been told, you cannot treat contractors/temps the same way that you treat your employees, otherwise they can claim that they were in fact employees the whole time (which usually comes with an expensive lawsuit and a visit from the IRS). In order to ensure that contractors/temps can't claim to be employees, it's not enough to not offer benefits. You have to effectively create a two tier system with FTEs at the top and contractors/temps at the bottom.

Not arguing whether it's good/bad, but it comes with the territory in the current landscape. You gain in flexibility/autonomy but loose in benefits/treatment.