You are correct that the end user controls what is ultimately sent to MS (or whomever is probing). However, if they are doing something like plugin enumeration

You are correct that the end user controls what is ultimately sent to MS (or whomever is probing). However, if they are doing something like plugin enumeration or using a java applet to go a little deeper, you then have to know what the request looks like and what info they are expecting. You then have to intercept that request and send back the expected data It's possible, but a little harder and more time consuming than flipping the user agent.