Normal
The problem is that things invariably play out in the same way: people who have an affiliate account will often try to push things as far as they can to the limit on what would be considered spamming, particularly if the revenue-per-link is low. And the problem of that (as well as visual clutter of posting-a-link-whenever-it-can-be-construed-as-peripherally-relevant) is the moderation overhead required to determine "is this spamming?". It's an annoying case of the trouble caused from abuse by a few outweighing the small benefit those not abusing it would receive.