I cannot tell you how many sites I find on the web with Google AdSense ads where it is almost impossible to detect the difference between the site's navigation and the ads. They are so blended together that often the AdSense link units look exactly like the navigation links on the site.
Why is this done? Well, to increase a site's click through rate and earnings on those ads - to make the company more money with Google. Of course, this is rarely ever helpful to an end user.
Now, people are complaining they are receiving letters from Google about rejections of their sites due to "difficult site navigation." In my terms, that likely means tricky or deceptive navigation, like in the cases I mentioned above.
Forum discussion at WebmasterWorld.