I can agree to a certain extent that the ACRTAG choice is in the hands of publishers, but still, making it default is a somewhat heavy statement from Valve. (also, if I didn't read SteamDB wrong, CS:GO currently has ACRTAG set to "no", so even a Valve games doesn't allow cross region gifting).
But I don't really agree with your definition of region lock.
I mean, it being a term coming from the olden console days, for me is simply a binary answer at the question: "will it run on my console if I buy it on playasia?".
This said, you can see that a situation in which I buy a game from Steam Asia (or RU/CIS, etc.) and that game won't run (or install) on my Steam, well, that's region lock for me.
Ok, back then it was mostly because we really really wanted to play, say, Densha De Go, but in a smaller scale it was for price reasons too.
You're English so it may have never occurred to you that from Italy (but I guess from other EU countries too) we used to buy games on Play.com because they were WAY cheaper. I mean, half of my old-old-gen purchases came from Play.com, and they were only possible because there were no region locks between UK and Italy. Today, we replaced UK with RU, but everything else is basically the same.
EDIT: links to Play come out automagically, I didn't mean to spam...