Sure but chances are that 90% of the users don't know how to do it. Also it's a good warning for the users. It's primarily not for protecting the torrent site but for protecting the users anyway.
I have no idea if the the trackers are doing this or not, but you could use TCP/IP stack fingerprinting. It's a bit costly, so I doubt it's in use. But it could be one way to achieve it. It's certainly not 100% accurate, but my experience with nmap is the detection works surprisingly well:
