I was going to post an example to help people better understand. . . but the hostility is pretty high and I'd rather not become figuratively lynched. It was a great example too.
http://www.umuc.edu/library/libhow/copyright.cfm is an alright place to understand copyright.
Look at everything regarding fair use as well as when you legally need permission to use a work.
Not sure where Eskii is 'borrowing' that info from, but here's a legit source that I hope isn't outdated (it says 2013 on the bottom, so I assumed they updated everything).
Most cases of plagiarism are considered misdemeanors, punishable by fines of anywhere between $100 and $50,000 -- and up to one year in jail.
Plagiarism can also be considered a felony under certain state and federal laws. For example, if a plagiarist copies and earns more than $2,500 from copyrighted material, he or she may face up to $250,000 in fines and up to ten years in jail.
Additionally, unlike what Eskii said, you have a significantly low chance of actually getting sued (in this instance), especially considering no monetary value is being held in the property being plagiarized, and additionally, the person who plagiarized is not gaining money that the original author would otherwise obtain-- to make it clear the purpose is non-profit, and, assuming the content plagiarized is no longer active, the plagiarizer is a) showing it to a new audience and b) not competing with the original work. The RP in question is probably not published, and if it's old than it's out of print and therfore leans towards fair use. These all give ounces of leeway, but they're OUNCES, and that's with the law of the U.S. not the law of RPG!!
I hate it when my information is outdated...