People used idlers to get weapons too.
It's not ludicrous. You're spoofing a connection to the Steam servers in order to get items even though you don't have the game running. Try doing that in a game where items and drops actually matter, like like WoW. Run a program that connects to the WoW servers and manipulates game data in order to give you items and see what Blizzard does to you. It's hacking, plain and simple.
It's not even a question of idling vs. playing. Read the TF2 blog; this crackdown is only on people who use external applications, and they explicitly say that using external applications is where they're drawing the line. And you know what? Good on them. I'm honestly surprised they didn't hand out VAC-bans.
As for warning people, what exactly would that accomplish? What are they going to do, make a blog post telling people, "We're going to remove your ill-gotten goods in a week, so get ready?" They're certainly not going to say, "Hey guys, you should stop using the idler programs, or we're going to remove your items," because that doesn't really punish people who've already cheated to get their crap. They're not going to come out and say, "If you already got away with it, we're going to let it go, but from now on don't do it."
And when it really comes down to it, if a player is using the idler program, he knows he's doing something they consider wrong, but he does it anyway. Then he complains because he got caught and punished? Screw that.
And for the record, I think the drops system is absolutely stupid and terrible. I have been playing regularly again since they instituted it and I don't have any hats besides the halo I got yesterday. Do I care? Actually I do; some of the hats are really cool and I would like to have them, but in the end, they don't matter that much. As for unlocks, I'm pretty sure that any unlocks I actually like to use I got through achievements, and if there's one I don't have that I want, I could just try to earn it through achievements, or hell, find it through the drops system during the course of regular play.