I'm well aware, IGN or Gamespot do it because they're the original publishers of the video and/or they have permission from the game
They ask permission for fair use, if you don't then your video will probably get flagged for violating copyright material that belong to other sources, and then taken down.
Not to mention the irritating fact of having people reuploading the same crap videos other sides have pretty much covered, is self explanatory how it can become spam, like literally crying for attention and views.
Creating content to be publish on YT = Good.
Reuploading what others have done = Spam.
Putting a watermark is simply a sign of the video being on your channel, you cannot get a copyright strike for such actions, using a copyrighted song or audiovisuals without permission will put you at risk.
There is so such thing as copyright for watermarking a video, i spoke with Machinima about these things and they said it's allowed on Youtube.
You're wrong once again Samus.