But maybe Yahoo! has research showing that Yahoo! users don't always know when they are searching. So when they do a search for Google or Google.com at Yahoo! Search, Yahoo! needs to remind them, that they also have a search engine. Sounds kind of dumb, but trust me, it makes sense.
What is interesting is that Yahoo! does it for Google keywords but not for MSN Search, Ask.com, etc.
How did I notice this? Well, I noticed a drop in traffic from Yahoo! News and figured it had to do with Yahoo! News vertical integration on Yahoo! Search. Guess what? It was. Google now doesnt come up in news results always on Yahoo! Search, instead you get Yahoo! telling you not to use Google.
Forum discussion at Search Engine Roundtable Forums.