Since when spam needs to be explicitly forbidden?

It goes without saying, in my opinion. This is a Q&A site, there are advertising possibilities on it, it's pretty clear that questions and answers are not the places for people to advertise their products.

It would be also useless to make such a line in the FAQ. It won't prevent any spammer from posting their stuff. I don't think there is a need to be "fair to them". 

Maybe your situation differs from Super User in that you get more "corporate" advertisement, but the effect remains the same. There is link for advertisers at the bottom. If they just want to spam their product links, it shows that they don't care and just want some free exposure.