Someone googles "free VPN" so they can watch region locked videos and now their connection is a part of that network too. They may or may not realize that this is the arrangement.

Or his kid uses a free game with a lot of grinding, not knowing that the child's phone is now technically part of a botnet.

I would like to know more about this. Got a source?

It's hard to find a source that isn't paywalled, but fortunately it's covered in a paper: https://www.xiaojingliao.com/uploads/9/7/0/2/97024238/ndss21...