Every single App Store out there uses "free" to refer to propriety software today, because it's free.

“Free” as an adjective isn’t the issue. The issue is the phrase “free software” being used to refer to things other than free software. And afaict, no app store uses the term ”free software” to refer to non-free software.

The iOS App Store refers to “Free Apps.”

Google Play doesn’t call it “Free Software,” either; they just use it as a category / filter, e.g., “Top Free.”

There's a reason many are … starting to refer to such software as "libre", not "free"

Your conclusion is incorrect - this is because when used outside of the phrase “free software,” the word is ambiguous. “Software that is free” could mean gratis, libre, or both.