You're dismissing the work load involved in implementing such things with an airy wave of the hand - it would be considerable, and in my estimation would probably at least triple the hours that would have to be spent by the admin team - someone posts a "borderline" link, so the team have to try to weigh up whether it's allowable or not - there are a shower of pms that will need answering, WW3 breaks out in the thread involved......... and that's just one link! (all this can take hours) - some people suggest links would be fine if the poster had no connection with the company - how, short of employing private detectives are you going to find out?
As you can see in this thread alone, there are widely differing opinions on everything, and every single posted link could be a powder keg - if there's a flat ban, no problems, you can't do it, and I really can't see the advantage of having commercial links at all - for all those whingeing about people hitching free rides, commercial links on a forum are the biggest freebies of all - (to increase Google rankings) .........