As he already said, it's called The Prisoner's Dilemma. The situation where everybody defects (everybody has a barker) is worse than the situation where nobody defects (barkers are unheard of). But the nobody-defects situation is unstable, because if Store A has a barker and Store B does not, then Store A will be swimming in money and Store B will go out of business (or, in reality, be forced to hire a barker themselves).

Probably the clearest example I can think of is doing a no-adblock Google Search for "Coca-Cola." The first result is an ad, put out by the Coca-Cola Company. This might not seem to make any sense, since the first non-ad result is an identical link to the same website, but imagine what Cott Corporation or PepsiCo would do if they could by the first result for their competitor...

And then there is me going into the only restaurant without a barker, because it was the only one I could check out the menu of without being hassled :)