I’ve seen a few reasonably recent US releases (not Disney, though) with burnt-in English subtitles for any theatrically subtitled bits – I’m not sure how unusual that really is. I’d imagine releases in other language markets, even within Region A, might get different treatment, but I have no proof of this. It’s also possible a second set of soft subtitles in addition to the burnt-in subs is how these films are treated internationally. Certainly that’s how the SDH subtitles work in these instances.
Just thinking out loud – not providing justification – but burnt-in subtitles might make multi-format distribution easier. Forget the Blu-rays, consider streaming services. Each service has its own (often partially-baked) subtitling implementation, and it’s doubtful any of them would be able to resemble the theatrical appearance through soft-subs. So you give the streaming services burnt-in subs on your video, and now they’re good to go. Same for DVDs, which, believe it or not, are still as popular a format as Blu-rays. DVD subtitling sucks, and you simply have to use burnt-in subtitles if you want them to look even half decent. That’s not to say they couldn’t use two different digital masters – one with burnt-in subs and one without – but I’m saying that there’s a pretty clear need for the master with the burnt-in subs, and someone would have had to convince a PHB at Disney not to just go ahead and use that one for the Blu-rays too.
Also, doing it “right” (burnt-in, but on a per-language branch, handling localized crawls as well) would have involved a lot of seamless branching. IIRC Disney pushed seamless branching to its limits not as a localization tool, but as a copy protection scheme, and they ended up with lots of upset customers and returned discs. Maybe they talked about localization, heard the words “seamless branching”, and bailed.