Until you have a problem, and don't have a receipt, and now you're out your money with a defective product and nothing to do except - buy a new one, I guess?
And I'm not sure what makes you think the person buying the thing is doing any better financially than the person ringing them up. Working class people shop too. You're letting your Poli Sci 101 take on class and economics cloud the fact that it's most likely another working class person getting taken advantage of.
The relevant parties aren't the cashier and the buyer. They're the cashier and the business owner. The whole scheme is meant as a deterrent, not as a remedy. If the buyer observes that the cashier isn't ringing up the sale, and the buyer challenges the cashier, the cashier will say "oops!" and proceed to ring it up correctly. That's because the cashier has nothing to gain by continuing the deceit at that point; either he's just letting people walk out without paying (thus risking being fired for no gain), or he's back to doing his job of collecting revenue in exchange for his wage. In any rational play of this game, the buyer ends up the same -- paying what he owed for the sale.
This assumes, of course, that everyone is behaving rationally. If you believe grocery stores are evil, as the parent comment suggests, I suppose you also believe you have license to make up your own rules.
Temporarily embarrassed millionaires eat my ass.