3) some people on default trust even make a hobby of marking red on people who haven't ever traded with them but with whom they simply disagree (see, for example, Vod vs MSCorp Key Sellers; or Quickseller vs. people who refuse to do business with him (
https://bitcointalk.org/index.php?topic=1023038.0, inter alia))
I don't think they're giving negative trust to people simply because they enjoy doing so which is exactly what a hobby is.
They give negative trust because they feel that a person has bad motives and in future he might scam others, they're doing this to protect other users, not because it's their hobby. And do remember that they're not moderators, they're simply doing a public service trying to keep naive users safe from scammers.