Source: http://www.calrest.org/tips-for-protecting-your-unemployment...
It also has to do with morale. If Jim just disappears and he's a FTE, that makes people nervous. They might start looking at a different positions. If Sally the consultant is gone tomorrow, oh well. There is often an "us" v "them" relationship between FTEs and consultants. It's not necessarily antagonistic, but it's not as amicable as FTE to FTE.
You can do X for no reason != You can do X for any reason
i.e. they can fire you when they wish to, but if you can demonstrate that the reason for firing was illegal, then it the firing is not for "no reason", but for an "illegal reason."
What you're trying is in UK parlance called "constructive dismissal" and it's not good; many jurisdictions make this risky.