Don't forget stealing all of the Dota hero/item concepts from said forum. For example, Warwick and Teemo were fan made concepts from the dota-allstars forums, iirc.
The website dota-allstars was owned by Mescon, and it was acquired by Riot when he was hired by them. At that point, it became their property and they were free to do whatever they wanted, and they chose to display an ad for their main product. They then sold the website to Blizzard.
How is that sleazy?
I hope you aren't offended by this but all this sounds like some incredibly stupid MOBA/e-sports drama.
"Sleazy" isn't necessarily "illegal." All you have pointed out is that they were able and within their rights to do what they did. Sure, nobody disputed that. The discussion is about the morality of the move.