Another reason I think some users plagiarize is because they think they can't write perfect English in the boards where English is allowed, so they go ahead copy and paste every article they come across, posting it as their very own.
I believe this is true. Their inability to reduce their thoughts to writing is a problem. That feeling that you have a good idea, but in writing you deposit rubbish, this might lead to plagiarism in order to standardise.
But I think this problem can be solved by typing one's original idea in their native language, afterwards use a translator to change it to English.