They do have a reputation for being sweet, which is kept so by people not breeding those which aren't.
Every breed, no matter how sweet its reputation is, will occasionally produce non-sweet animals.
The same is true for whatever else the reputation (or ideal temperament according to the breed standard) is... An aggressive breed will occasionally produce sweet animals. A non-brooding breed will produce broodies. Etc.
In order to preserve the ideal temperament of the breed, those that don't possess the right traits are culled out. Ideally. Unfortunately many people don't do this, so vicious Silkies aren't uncommon.