Well, a post to these forums for the purpose of (or the primary function of which is) distributing, or touting, anything from which you make money (or, for that matter, to drive traffic to your site) is a type of forum spamming.
These invitations are basically "mega promo codes", and you are paid $97 when they are used, so posting here to distribute them (circulate them, offer them to others, etc.) is no different than posting to distribute a promo code or a referral link.
Think for a minute how the "noise to signal ratio" of these forums would increase if everyone did that. To me, they are the same as promo codes, referral links, etc., which have been considered acceptable only when they are:
1) Not part of the content of the post.
2) Restricted to inclusion in your signature.
Another thing to consider is the warning that accompanies the grant of those invitations:
I surely would not be offering them to anybody unless I was 100% sure they would not end getting disabled!
--DreamHost Tech Support