It's not meant to. It is breaking rules.
The only live things you are allowed to sell are inverts-officially under the category of feeder food and some aquatic animals.
Perhaps it depends on the poster's country or region? For example, in Ontario Canada, no ads for live animals are allowed on Craigslist. We have something called Kijiji, which is basically the same kind of classified ads site and where animals are allowed to be offered. Strange though, to allow one and not another...
It's not allowed. It is in the UK. Usually these ads get spotted earlier on other forums, and get reported before the ad finishes. Bizarre that the first forum to report this was a US one.