Twitter Breaks Copy/Paste on tweet pages

Go to any individual tweet like the link below and try to copy the text. It won't let you. This behavior doesn't exist if you try to copy from the front page.

https://twitter.com/#!/fredwilson/status/185755195735687168

Why do sites break standard behavior? Is it really that unreasonable that users would want to quote a tweet? This is just as bad as news sites that alter the text copied.