Yeah, carrier unlock and OEM unlock are completely separate and have nothing to do with each other. People will say 'unlocked' and mean carrier unlocked because most people don't know what OEM unlocking is so it can get really confusing when trying to buy second hand and explaining the difference. Its a really common misconception that has been an issue for over a decade that's why I always try to explain they have nothing to do with each other
I don't know any other carriers that do this but I wouldn't be surprised if more did. I never recommend buying from a carrier since most just carrier lock their phones anyway so its just more hassle if you decide to switch carriers.
I find buying from the manufacturer works out cheaper than from any carrier where I am. Its also better for privacy since I don't have to deal with bill pay and can just pay cash for PAYG without giving any personal information and can switch to whoever has the best deal without faffing about, just get a new Sim card and I'm ready to go