Can't they just put disclaimer, "If you insist on submitting your idea, oh well, thanks. We might agree to realize your idea or part of it, but you must know that we can't compensate you in any way."?
The problem is that there's not a central hub or gate through which all concepts and submissions must pass. It's not like a website, where you simply post a disclaimer or agreement that everyone must encounter before submitting something. People can theoretically try to submit to TV networks in any number of normal to bizarre ways: email, snail mail, stalking a development exec and accosting him at Starbucks, throwing a package over his fence at home, etc. You name it, and it's happened.
It's nearly impossible to retroactively disclaim things submitted through such varied means. So the safest course of action is just to send things back, or disregard them altogether. If someone's really written a brilliant masterpiece, then he should get an agent and go in through that door.
It's an imperfect system, but it's a system born of necessity. Even big networks typically have scripted development staffs of no more than 10 to 15 people (sometimes considerably fewer), all of whom are spending 100% of their time working on existing projects, or hearing pitches from agencies and producers. These people don't have time to read everything that comes their way through the mail, even if they wanted to, and even assuming that more than 1% of what comes in unsolicited is actually worth reading. And none of them wants to bear any legal liability. (Imagine greenlighting a show about superheroes, then opening a random envelope and finding a pitch that's very similar; talk about a potential "Awww, fuck" headache moment).
This was exactly the kind of disclaimer on the thinkgeek 'submit t-shirt idea' page, at least 6 months or so ago. I would link to the page that had it but it seems it's now behind a user login page.
