I see this argument pretty regularly, but I don't understand it. Of course you can run ads in RSS feeds; you can put whatever you want in an RSS feed.
It doesn't help with auto discovering rss tools, but it does handle the person who right clicks and copies the url of the feed to put in their feed reader.
If it were simply about only showing "smart ads", Reuters could just continue to publish headlines and links without content in their RSS feeds, but they're not.