It's really rare for an escaped "pet ferret" population to boom on it's own, since most ferrets purchased in the pet trade (i.e. not specifically purchased for breeding) are already spayed/neutered. This isn't the case for most other pet species.

IIRC, this is done because female ferrets who go into heat can/do die if they don't mate and male ferrets will become quite hostile when near a female in heat.