What's inauthentic about paying for a product with social karma? It's not like this system is duping people into tweeting out ads. Everyone on Twitter knows what it means to tweet, and they will understand that paying with a tweet means that it goes on their feed, and that their followers will therefore have a {positive/negative/neutral} reaction to it.
And, even if some rooks do not have that understanding, it will not ruin things for those of us that do, because we will unfollow them.
If enthusiasm isn't authentic .. who's going to take notice?
If the generated buzz isn't actually generating interest organically, then this scheme is just about link building .. which is equivalent to spamming imo.