This is a grey line. So, watch your step. You can be a competitor forum and be allowed to link, but if you pissed off that competitor site [mod, admin or owner] not only will you not be able to link the site because the owner censored the link via filters, and will ban you pretty hard.
For example, a competitor forum came over to my site and registered. What'd he do? He registered with a username that has his site name, and then spammed the hell out of my forum with links in his sig and the posts itself. So, I banned him immediately, and then killed all his posts. All of it.
I was pending a negotiation that went sour with him, and he decides to spam my forum at the same time. Jerk.