A complex society relies on people avoiding stepping on one anothers' toes. You can't always rely on the Refer[r]er header to block access to resources, either.

Just don't freaking do it. It pisses people off to no productive end (as opposed to personal scraping, whitehat access, etc., which also piss people off, but serve a useful purpose).

Also, copyright provides more rights to creators than just restricting copies and distribution. There are also public performance rights, which I would argue are violated by embedding another site's resources into your own site without permission.