If a company wants to open up an account and put their products on there in whatever fashion they like, then I'd say go for it. People can then choose if they want to look at it. What I don't like is when adverts are flashed in my face without any choice. I guess they could advertise via comment spamming though, which could get annoying... but then I suppose it wouldn't be long before their rep is so low that the spam would be hidden.
I don't think we should censor by banning certain types from getting accounts. We have the freedom to choose not to view those accounts. I'm not up for no choice about viewing advertising though, because takes my freedom away.