Stand corrected on that. Dont know why its not.
I think the reason is that penalty points are (supposed to be) for safety related offences only.
Although it may be extremly annoying to other road users, going down a bus lane isn't putting anyone's life at risk.
I could be wrong on this point, as I can't point to a specific reference for it.
I'd tend to agree with it though, as if they start being handed out for non-safety related issues, people will just get cynical about them and why they're there, in the same way they do about guards pulling people for speeding on the safest rather than the most dangerours stretches of roads.