Individuals who take part in this practice, the so-called "domainers", and many of whom are registrars, are able to bypass registration fees by taking advantage of a "a five-day grace period". If the re-registered domain has traffic and warrants the $6 registration fee, they re-publish the website with pay-per-click advertising. Otherwise, the domain is released and the cycle starts all over again.
ICANN is currently considering a 're-stocking fee', with a vote to be held later this month. The grace period for .CA domains, managed by CIRA, is 7 days. The CTB post also notes that domain tasting creates a secondary market, and that it can "propagate cyber and typo-squatting".
Not everyone is aware of this domain registration monkey-business, which is why I'm passing it along. The moral of the story is, of course, never EVER let your domains expire!