It's impossible to police.
That's not entirely correct.
I don't know anything about computer b.s., but do know that some sites have the ability to decipher what other programs you're running on your computer while you're playing on their site.. '&' if you're using PE it'll often get detected.
I'm also aware that PE will upgrade their software in an effort to have it go undetected but then it's a back & forth battle as sites do upgrades/fixes followed by PE doin' same, etc. etc. etc. If you check back through their forum you'll see times when they've said to proceed with caution until they come up with upgrade/fix (< this is quite awhile ago that I've checked this out so I'm not sure how it is these days).
I've also had a couple of friends (2) who've downloaded the '5day Free Trial' & then within 24hrs. have had the site contact them via email, instructing/warning them to remove PE or their account will be closed. I also had it downloaded at the time (free trial) but then uninstalled it before even using it after seeing what happened with friends.
(fwiw, there was some other program I downloaded a free trial with (I forget what the name of it now (was yrs. ago).. 'Spy something'..or 'Something Spy' & while using this I also had pokersite contact me about it, letting me know I'd need to uninstall it if I wanted to continue playing on their site)