On Twitter's blog post Twitter Blog: Pushing Our (Tweet) Button they are typically straightforward about their reasons for this development.
Another nice development is that it uses t.co for url shortening, but in the tweet you see a snippet of the real url - good for knowing what website you're about to visit. More information here: Twitter's URL shortener, t.co.
Twitter's implementation resources:
<a href="http://twitter.com/share" class="twitter-share-button" data-count="horizontal" data-via="swinny">Tweet</a><script type="text/javascript" src="http://platform.twitter.com/widgets.js"></script>
<iframe allowtransparency="true" frameborder="0" scrolling="no" src="http://platform.twitter.com/widgets/tweet_button.html" style="width:130px; height:50px;"></iframe>