It's not asking something of a user, it's forcing them. If this was optional it would be asking. I think it would be great if it was optional and showed at the end of the video, why would you tweet about a video you've not yet seen?
I can't ever see a use case where it is OK to force a user to syndicate your content before viewing it. It trying to force virality -- and after a while users will stop watching videos, negating the effect if not harming the content in the long run. Think "Expert's Exchange"