I didn't take it that those videos were illegal. But they tried to trick the criminal into watching some videos with low view counts that they would be interested in, and then planned to ask youtube who viewed all of those videos during that time.

Like an ip gathering honeypot website. Except they probably assumed the criminal would be too smart to click a link to some random website. But they would visit some youtube links. And then they knew Google would roll over and give them the info of all viewers.

It's a scummy plan, but mostly because Google will give away that info without putting up any fight.