The Credit Card companies employ highly sophisticated fraud tracking systems and can quickly track a breach back to the Merchant. Even a handful of Credit Cards can lead the Credit Card companies back to the Merchant.
In the event of a breach, it is important to inform customers to prevent further identity theft issues and to reduce the levels of exposure the Credit Card companies have.
Once the details of a breach are released, and they nearly always are, the press, the bloggers and consumers all email, write and talk about what has happened and why.
A data breach could have occurred because of the concerted efforts of a hacker but the perception will be the Merchantís security or procedures have failed. Consumers are become more demanding and are likely to take their business elsewhere if they feel their personal information was not adequately protected.