Would something that transparent actually stop you from having grounds to sue, though? If you documented that they rescinded an offer, then you threatened to sue before they hired and immediately fired you, I don't think them doing that would invalidate your case at all.

I doubt they'd do that.

First, "at will" employment is pretty complicated. It's not as cut-and-dry as it's made out to be. Besides, this is a detrimental reliance case: you're arguing that you relied on an offer that was fraudulent, and if they fired you in the first week and couldn't show that it was part of a business layoff or due to criminal behavior on your part, you'd still have that argument.

Second, once you rescind an offer, that person's pissed. Letting that person into the building, even for a week, is not something a risk-averse company is going to do lightly. Third, companies would have all sorts of internal-publicity/morale issues in doing this: hiring someone for a week just to can him on an obviously phony performance case. Fourth, companies care as much about disparagement as about lawsuits, which is why you can often get a settlement even if you don't have grounds to sue (or, more commonly, the suit wouldn't be worth the time put into it). Of course, you shouldn't threaten to disparage in a severance negotiation, because if you don't know what you're doing you can end up unknowingly committing extortion, but it is a concern on their side.

Companies are risk-averse and want to protect their reputations and avoid headaches. Once they've rescinded the offer, a 3-5 month severance is easier than taking the risk of letting you into the building. What if your boss and team like you (and, in the first week, why wouldn't they)? Do they really want to deal with the morale mess of firing you in the first month?

Of course, this doesn't apply when offers are rescinded for legitimate reasons (whole-office shutdowns, massive layoffs) but that's not what we're talking about here.

The problem is, if they're doing it right, they won't even know you failed background because of your credit score. Companies outsource background checking to protect themselves from exactly this. They don't even want the potential liability of knowing why you failed the check. They pass your name and address as parameters to the background check company, and the background check company just returns a boolean value. The offer is always conditional upon passing background, so if you don't, the offer is null and void. You don't get to know why.