Generally speaking, hosts are banned for behaviour that is considered unprofessional and damaging to the reputation of the board and/or the industry. Reasons can include (but are not limited to, to my knowledge) spamming, hostile behaviour within the forums (insulting people, making intentionally controversial statements, that sort of thing) and other such behaviour. The reason URLs are blocked out is to avoid people, search engine spiders, etc. visiting the offending site and giving it more popularity than the company would deserve.
This happens (at least that I've seen) very rarely, and it takes a lot for BC, Chicken, and the other mods to take this action. If the company name or a variant of it appears in the thread (e.g. C|Host), do a search on it (and any variants you can think of) and chances are you'll find at least half a dozen threads on the company where customers have been treated in an unfair manner.