They are also surprisingly effective because they often show things that I might actually buy (especially when it's on sale, which is precisely when they show ads for it).
No, that's not an excuse because Steam is also a launcher to play your games. If the store was completely separate then sure it would be OK to promote games being sold in the store there.
I'm very anti-ad, but if there's one situation where I don't have a beef with it, it's the Steam app.