The url that you copy is not unusable, it's a redirect url.
Which means you end up having to open the URL first yourself just to copy and share it. And that doesn't even always work, since URLs triggering downloads don't update the URL in the address field (at least in Firefox).
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&c...
I'm sharing it with you. Is this unusable? Did you not get to the news article I was sharing with you by clicking on the link? So what's your point?
Same reason that URL shorteners are bad UX. Source (creator of Delicious): http://joshua.schachter.org/2009/04/on-url-shorteners