i'm rarely at youtube so i can't say for sure if the button is there...but it is part of IE7 Pro (the ability to download media in that way)....
actually explicitly state that:
[quote name='http://youtube.com/t/terms']A. You agree not to distribute in any medium any part of the Website, including but not limited to User Submissions (defined below), without YouTube's prior written authorization.
C. You agree not to access User Submissions (defined below) or YouTube Content through any technology or means other than the video playback pages of the Website itself, the YouTube Embeddable Player, or other explicitly authorized means YouTube may designate
B. You may access User Submissions solely:
for your information and personal use;
as intended through the normal functionality of the YouTube Service; and
"Streaming" means a contemporaneous digital transmission of an audiovisual work via the Internet from the YouTube Service to a user's device in such a manner that the data is intended for real-time viewing and not intended to be copied, stored, permanently downloaded, or redistributed by the user. Accessing User Videos for any purpose or in any manner other than Streaming is expressly prohibited. User Videos are made available "as is."
F. You agree not to circumvent, disable or otherwise interfere with security-related features of the YouTube Website or features that prevent or restrict use or copying of any Content or enforce limitations on use of the YouTube Website or the Content therein.[/quote]