Oleksandre, you got an official response that this is by design. I explained why. This is a landing page, not a separate product, and the description of the plagiarism feature clearly states it's not for policing:

"...Our free plagiarism check will tell you whether or not your text contains duplicate content. Our Premium plagiarism check highlights passages that require citations and gives you the resources you need to properly credit your sources."

We continued our discussion in this thread https://news.ycombinator.com/item?id=29933516#29947146