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...