because it's code in a browser. if the author didn't want us to look at the code, why would they let us download it?
Ability to do something does not mean it is ok to do it.
As a musician, if I posted a piece of music to a site that permitted it to be played in their Flash player on the site, this does NOT mean I want it to be distributed does it? Unless I explicitly stated that it was alright to distribute it.
EDIT: I didn't mean this to come across as snappily as it did btw!
just because it's viewable, does not mean you are granted the rights to do whatever you want with it.