Always read the TOS of any site before you use any of their products or services that way you will know what the deal is and you will never have to worry.
It might be for some malicious websites, but the best secure plagiarism checker I know is copyscape.com.
I have ppl that write content for me - I myself use copyscape as well. However, once I get their article - before I place it on a site - I change a few things myself. I really don't trust copyscap 100%
The accusation has certainly raised a few questions in them minds of the users i nregard to the credibility of the plagiarism checkers. But, I can assure you that quality plagiarism checkers like copyscape won't ever let you suffer from such issues.
