I'm guessing you've not seen the cash flow of a small item store? There's a significant percentage of money you end up "losing", regardless of payment system. Actual counterfeit bills, con artists confusing clerks to give wrong change, credit card companies pulling back payments even weeks after the fact, even debit payments can be denied if the store didn't check the signature... It's useful to realize that bitcoin isn't competing with perfect systems: all of them have these hidden costs, and so far the costs related to bitcoin seem small in comparison. I have no idea if bitcoin will be used in day-to-day shopping at some point but this issue does not seem relevant to me.

As to your example: doing a double spend attack on a $250 purchase (that is handled by a competent payment processor) is just not financially sane. So yes, I absolutely believe a store that handles such small amounts will accept 0 confirmations. The tiny risk might be taken by the payment processor or the store, that remains to be seen.