It's actually not though. That's oversimplification. The fact that you can't spam your ref links doesn't mean you can't post them anywhere. There are few threads out there with ref links in them (I have few of my own), you can get away with it, generally if: 1- there's a purpose to your post other than promoting ref link/or using ref link is justified; 2- you post in relevant board; 3- you make it clear to everyone that you used referral link (you're not trying to conceal it etc.).

But yes, in 99.9% of cases - it's not allowed. Reddit is much better for that, if you do it in a smart, non-spammy way.