This really makes a lot of sense. You have pretty overt ads displayed, and deletable, within a tab devoted to ads. It's pretty much the best scenario imaginable for an ad supported service.
One could argue that signing up for a Google service is enough to form a "business relationship" with them. Just like how if you buy a new pair of rocket skis from ACME, you're likely to get promotional email from them featuring sales on anvils.
The promotion tab is where you get promotional emails from services that you have OPTED IN to receive emails from.
So is it far game if the Viagra emails only appear in the promotion's tab?
Google is sending spam (unsolicited emails). That is all there is s to say about it.