This would speed up the perceived page load to the user.

More importantly this would prevent problems of loading pages in China where twitter is blocked. Right now you have to wait a minute and half for the javascript to time out before the rest of the page loads.

here is the documentation on the issue from twitter for the follow button I would assume the tweet button would use a similar technique:

https://dev.twitter.com/docs/follow-button#how-can-i-load-the-javascript...

Comments

I also found this blog post for the tweet button:

http://blog.wyrihaximus.net/2011/05/loading-your-twitter-tweet-button-as...