Lack of access to files in application/third_party/

The full links to css (generated) are correct. Trying to view the css with Chrome + Web Developer, the result is a "403 Error".

I moved the template from "application/third_party/template" to "assets/template" and now it works. That's okay.

But why it does not work in "application/third_party/"?
Is there anything that prevents the css reading from here?

Thanks so much.