The posts they got banned for were probably written years ago at a time when the users were relatively inexperienced. The BT unofficial rules were written sometimes in 2014 and if I remember correctly there were cases where users got banned for posts in 2014/2015, Cellard is one of them i think.

Probably a combination of different things that led to the copy/pasting.
Not knowing the rules.
Administration didn't put so much emphasise on plagiarism back in the time because it wasn't as wide spread as it is today.