Not quite true - they require that you not sell Steam keys for less than you do on Steam. They still don't even stop you from doing giveaways or participating in bundles. It's just that your typical prices on independent Steam key sales, for which they don't even take a cut, can't be lower than Steam prices. Also the seller sets all of these prices.

Given they're footing the bill for indefinitely hosting the games supplied via those keys, that's an entirely reasonable restriction.

This is coming from someone who is against capitalism and all IP law. The big problem with Steam imho is that Gabe Newell won't live forever and when he's gone the company could go public or go to some fail son who will tank it. I'm not even saying Gabe Newell is a great guy or an ethical billionaire, but he's been remarkably consistent in keeping Steam's business model running well.