An engineer friend of mine asked, "Why does Nikon use grease to lubricate?"

I'm guessing, as with any product, it would need to reach a certain level of frequency before any sort of recall or service advisory was issued. It may seem like many people have the issue, but those with the issue are likely an incredibly small percentage of sales, and Nikon probably feels like it is better to deal with on a case by case basis through warranty. Service advisories usually aren't done unless it is a safety issue, or affects a large number of products. Remember for every 1 person who makes an oil spot thread there are probably 100 happy users who don't even know there is something to check for. Again, I am not saying there are no problem cameras, but there may be a small enough number of them relative to total production that Nikon feels they are better off treating it on a case by case basis. That is the most likely scenario (as with any product), but we will probably never know for sure!

Funny you should post that Canon link, a quick read of these forums would have you believe oil is a Nikon-only problem (which is the other part of the issue - forum hysteria...).  Thom Hogan's results on the D800/E AF issue were a good example of this.  Of the people who wrote him thinking they had a problem, less than 20% had a real problem, and over 50% were "false positives".  He talks about how internet hysteria rapidly exaggerates an issue, which is still a real problem for the manufacturer.