You're right and wrong.
This has zero to do with the 1st amendment. Youtube is a business. They can show or not show any video they chose. Just because someone makes a video doesn't mean they have to show it. Frankly I'd say they should take about 90% of the crap on their site down.
But I digress.
You're correct in that it seems to be a coordinated 9/11 anniversary attack rather than caused by the video