I think everyone has their own ideas on this topic. Part of the discussion is of the nature of NSM's quote-on-quote "ownership" of the arrangements hosted.
Keep in mind the intent of the site is not to own anyone’s work, but rather to gather the world’s best collection of quality video game piano sheet music.
Based on this, NSM is more of a collection of the work of others' which is quality-tested. The arrangers voluntarily offer their work to be hosted under NSM through the submission process (and sealed with the NSM url at the bottom of the sheet). The fact that the work is not owned by NSM is clear through the possibility of having one's sheets taken down from the site, which is also addressed in the aforementioned quote source.
But then the question remains: if NSM does not own the work, who does? I'm inclined to say that the arrangers themselves retain ownership of their work at the bottom line, again, because they can technically have their work removed from the site if they so choose; if the work changed ownership to NSM, this would not be possible. An example of changing ownership is in the video game music industry itself: most of the time, the composer(s) write the music but it is then sold per contract to the developers. Once that happens, the developers have complete control of the music, and the composer has virtually none. E.g., if a soundtrack CD is going to be released, that's generally the developers' call and ordeal- completely independent of the composer unless they deem otherwise. This also then has concerns for self-plagiarism as well with future compositions for different developers, but I digress.
Ultimately, since the ownership then stays with the arrangers, each individual arranger is the one that carries the weight, final say, and opinion on this kind of thing. Since nobody makes money here though, it removes the legal liabilities that might otherwise be present; which turns it into prime flame-war material at best. That all being said, I'll now share my thoughts on this whole thing:
My Thoughts on this Whole Thing
Unsurprisingly, I like to see a bit of credit come back by those who take my material and post it on YouTube (or similar mediums for that matter). Even a slap in the description or beginning/end of a video is often way more than most people, and I really do appreciate that kind of gesture. In fact, done right, I actually would probably enjoy folks using my stuff in this way. Trouble is, most don't and those that might put something in the description will still just milk any praise in the comments as though they were the creators; leaving us actual creators by the wayside. It's kind of a case of 'just take it from our perspective': how would you feel if you were the content creator; honestly?
However, regardless of what I'd like to see happen, I specifically will not go after anyone who takes my stuff like this without credit; and no, I'm not afraid to say so publicly. This is for two main reasons: 1) I'm a bit of a philosophical purist who believes in music first and foremost for people- if my work can do good for others and their musical development/aptitude whether it gets traced back to me or not, I'd rather have that than just withholding my work entirely. Bad business practice, yes. But I'm not making money here anyways so I might as well be charitable as it were. 2) Those who actually own the work that I arrange and work with could, likely and at any point, march themselves in here and shut us all down because we don't have any particularly special clearances either aside from the aforementioned "trying to credit the right people in all things". We like to think that if anything, we're boosting the popularity of the games and the music through our work and make it very clear to people that we claim no ownership of any of the original compositions; but when push comes to shove we're really not any different than the YouTubers who take our stuff. Needless to say, we do our best to treat the official sources the way we'd want to be treated by those who use our work, but in the face of huge corporations not-suing-us-for-everything-we're-worth, I think the least I can do is not make a fuss when people take my already-backpacked-not-original material. That all being said, plagiarism is bad; don't do it.