I'd rather go the other way. It's not (or, shouldn't be) the business' responsibility to enforce civil law. I'd rather have them not do anything at all. The issue is between the rights holder and the thief. Barring an obvious marking of some kind, ala Olan Mills, the store is ill-equipped to judge one way or another. All the store is doing is offering an otherwise legal service.
All signing a piece pf paper does is absolve the store of any liability. They're not really doing it for the rights holder's benefit, so why go through the farce? Simply make the store not liable to begin with. Why do honest people need yet another annoyance in their daily lives when the end result is going to be the same anyway?
(I've since changed my mind from my post of a couple days ago)
When drawn into the courtroom, it will be a matter of whether they've done acceptable "due diligence" to avoid being accomplices.
Pawn shops have to show "due diligence" in avoiding stolen goods. Did they steal the goods? No, but they will be found liable if their normal business practices do not show some minimal effort to avoid them. It doesn't take some kind of draconian iron-clad procedure, like demanding to see the sales receipt for any item brought to them. They only have to do a little bit, like checking an ID and recording contact information. That is enough to discourage most thieves, and it's enough to convince most courts that they've done a minimal level of "due diligence."
It's the same here. Walmart is covering its own okole just enough to have a "due diligence" argument in court.

