I also know that some PTC sites have an option so that you can choose a type of device from which traffic is coming, but I still did not see someone offering an option of web traffic based on browser type.
The sense i see here is about exploiting those users who come with IE browsers, that's the only reason i see for someone to ask for IE traffic, do you know why only 6.8% of internet users use that explorer, because they don't know how vulnerable is that explorer, but if you go to exploit-db and search for IE you will find tons of exploits to bypass those users.