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.
It doesn't make any sense to me to get all worked up about "rude behavior". The internet is full of people who want to break into your box and take over your system. If you have a server facing the internet you have to expect that people are going to be ruder than you ever dreamed possible, and they are going to do this all the time. Complaining about it won't change anything; just fix your server so it doesn't do things you don't want it to do. I understand that there's this whole big social thing about "hotlinking" which has grown up now, and I've heard all the reasons people use for it, I just look around at the state of the internet and think it's a trivial thing to worry about. Just fix your config and get on with life. Browbeating someone else for using resources which are advertised and provided to the public for free seems like an amazing waste of time.