I think they did this to improve performance.
You see, they gave the tag a background using css, the background is done using a sprite to reduce HTTP requests and increase YouTube download speed, stating that they did this so that people do not steal their images - complete garbage.
In the next part, it can be a little difficult for me to explain, so I will try my best, sorry if I lose you here:
The image tag was used, since there really must be something to give a background, and having an image there makes sense (since there really is an image, it was just generated using CSS, not HTML), for example, they could it would be easy to put a div tag there, but it doesn't make sense to put a div tag there than an image tag (even if the img that they place there is an empty image 1px by 1px in size)
They could also do this in order to improve consistency, for example, they also used it on other sites of their site, see the youtube logo at the top left, the same method that is used except with alt text, here you need an empty image because in otherwise, alt text will be displayed there (you can get rid of the image using firebug to understand what I mean); here the image tag is used to improve accessibility without sacrificing speed (you get the speed of sprites and the availability of the alt tag attribute)
Hope that makes sense