They switch to LN, simple as that.
The only reason to accept zero confirmation transactions is to not let your customer wait for minutes or even hours after he has paid for his coffee, pizza, and groceries, holding him hostage till the tx is confirmed. Besides, in most cases, if we exclude tourist hot spots the ones paying with bitcoin are just a few, merchants would know them already as regulars, so what are you going to do, scam a grocery store of $50 and then never show your face again there? Not even mentioning the fact that the business can file a complaint for fraud?
It all comes down to a simple thing, will you lose more in revenue from customers, or are losses from theft higher than the profit, merchants always have to balance the pro and cons of everything in their business, and some will get scared after one-two thefts and drop it, some who have just regulars will keep doing so even with this change. Online merchants probably won't even care, they will definitely have the time to stop the shipping, and gambling and exchange sites will just all use the same cooldown some do right now where you're not allowed to instantly withdraw the sum you have deposited despite them accepting zero confirmation deposits and allowing you to gamble with them.