I think you're misunderstanding the roles of developer, publisher, and distributor.
If you look (like I just did) you'll find that Steam isn't region locking games, the publisher is. And yes, Valve is acquiescing, but we don't know what kind of agreements exist between Valve and the publisher.
It's ultimately the publisher's decision, although it's still an issue that affects such platforms.
In that particular case Steam itself was not disclosing this to users in those countries. So on release day in the US, when the countdown timer reached 0, those in region locked countries had the surprise of not being able to play their pre-ordered game.
It's simply another frustration that made customers wonder what they had paid for, with some trying to use a workaround like a proxy (not always successful), or even considering pirating the game in the meantime.
If you look (like I just did) you'll find that Steam isn't region locking games, the publisher is. And yes, Valve is acquiescing, but we don't know what kind of agreements exist between Valve and the publisher.