Just as @Pmalek has explained above, it is not double standard in any way, there are only very few things a user would do on the forum that would get them banned, the most popular being plagiarism, but scams are not moderated by the forum. You do business with others here at your own risk, and all that can be done if someone has sufficient proof of being scammed by another user is to bring the trust system into effect (by giving out negative feedbacks).
Do users even get banned for spamming, i think you'd have to spam for a pretty long time, write utter rubbish (in the wrong sections), till it becomes extremely annoying before you get into some trouble (maybe even a temporary ban at first).