In the old days, we just sent the content and let the client figure out what to do with it. If you navigated to homestarrunner.com and you didn't have flash installed, you just got a message saying to install it, or worst case, a broken icon in a frame.

We figured it out just fine.

Right, but the client side did a query to see what was available and gracefully displayed a message. People are now sending the truthy response of the feature's availability back. You could be doing that as a legit bit of logging, or you could be doing that to be an asshat and deanonymize the user. The browser doesn't know the intent of being asked about a feature. And yes, it has been figured out just fine as a way for making someone money by selling information about you