The more straightforward explanation, is that it would drive people to use Google Search, which RSS effectively bypasses.
And I still don't see how that even makes any sense: Googling something is an active search for a specific piece of content (even as broad as one site) and feeds of any kind are passive consumption of content which is brought to you. I can't imagine a universe in which someone loses their feed of content and increases their web search activity instead of switching to another (perhaps less-useful) feed of content.
If you set up a server-side aggregator, with keyword filtering (imagine a large list, something in the +thousands of words) and have your reader check for those words every few minutes, you're basically plugged into the search engine, completely bypassing their front end. You can suck up their results, then filter those yourself, again. Even when they give you a limited number of searches, you can still set up multiple smaller RSS feeds, then put them all in one big feed. Do this for multiple search engines and you never have to click on their pages again.
It's basically a free service with no return for a search engine, while they're paying for the processing.