Wait for enough confirmations where the payment becomes unlikely to reverse, which of course takes time and that's the more practical blocker for regular shops to accept Bitcoin.
They aren't doing that, in places I've seen, they just take your payment and let you go away with that. (I've never paid myself, but I've seen other customers do so)
If it's raw bitcoin, they couldn't even be sure that the transaction is a valid one (that the wallet has the funds in the first place, not even talking about double spending). I suspect they use some kind of third party like Coinbase, and that there aren't really using bitcoin at all (and just use Coinbase as a bank) but I'm not sure.
Thanks. I'm not sure I understood the answer given in the specific post you're liking to (or at least, I don't see how it answers my question). A following response linked to this story: https://www.ccn.com/bitcoin-atm-double-spenders-police-need-... which gives a pretty good answer.
"Digital cash" is really the best way to explain the uses of it. Cash is still useful as well. And, equally, don't fall into the cashless utopia "why would anyone want to use cash except criminals?!" trap either.