YouTube launched as a platform built upon viral sharing of content, which skyrocketed in popularity by the inclusion of an embed feature that allowed users to post video content on their own sites. Up until this point, videos had largely been quite expensive to host and deliver to consumers - with most sites protecting their videos through anti-hotlinking scripts and technology.
Enter YouTube, allowing users to not only post their content on the site but to also allow others to share it. Publishers flocked to the site, especially major publications, to help disseminate and spread their content. Others used it as marketing channel, even going so far as to allow others to share ads and clips for their latest content. Well it appears that much is changing, with more publishers opting to disable video embedding.
The growing number of publishers includes major political sites (Fora), educational channels (Discovery Channel and Animal Planet) and large news wires (AP News).
Profit is likely the reason that most are pushing towards disabling sharing of their content outside of YouTube. There's more to be made by having users funnel to YouTube and view the ads (of which the publisher receives nearly a 60-70% commission through Adsense for YouTube). Additionally, video embedding means that your content can appear anywhere - both a pro and con. For the stodgy publisher, who is most comfortable with traditional media channels, video embeds might be viewed as threatening since the user has more control over the content and the profit that can be made by promoting it.