Earlier quoted context omitted.
They have an implicit most favored nation clause which is by definition anti-competitive. Valve takes 30%. You can’t, in practice, sell your game on Steam and on another store at a lower price. That’s anticompetitive. Downvote me if you want. But I recommend reading the transcripts from the Wolfire Games antitrust lawsuit against Valve before you do! They’re not a good look for Valve to say the least.
I'm genuinely curious. If that's the case, how is it that I have bought dozens of games on the humblebundle store (for Steam) that were far cheaper than the retail price on Steam itself?
That said: enforcement on such things is not going to be 100%. Larger companies will either be purposefully ignores or make their own internal deals and contracts to follow. Some smaller games will slip through like everything else in life (Valve can still let Malware slip in once in a blue moon. I'm not surprised you can find some niche Japanese game sold for cheaper on DLsite or wherever).
But the point is that they can push thst in devs because of the monopoly. And that's how you get stuff like the Wolfire lawsuit when a few people do push back.