This dislike isn't arbitrary. My problem is that I wrote the paper. I don't want a third party to be able to use it to their own monetary gain without my permission. Being forced to do so left a bad taste in my mouth. When I tried to get in touch with TurnItIn to express my concerns, I didn't receive any response.
I can see the counterargument in your head already -- "But you use Google, don't you? How about Amazon and hundreds of other services that keep information about you." To which, I respond that I'm not signed into Google (prefer Bing or Duck Duck Go, at that) do not use Facebook and find Amazon's recommendations useful enough that it doesn't bother me. And I chose to use the rest of the services.
These companies also make their privacy policy and what they do with the information clear. With TurnItIn, the privacy issue wasn't clear when I looked. Would they ever sell the database? If they did, would they make a reasonable effort to protect my information? I was turning in a paper with my name and email address on it, after all. Tied directly to my college information, at that.