Glad someone else started the whole "Not speaking from any authority", so I'll follow your lead... (-:

In addition to said usefulness, it doesn't necessarily have to even be something that Microsoft may consider useful to keep as IP, and -- here it comes -- not speaking from any authority but I recall that there were some components that Microsoft may not have the complete ownership of in these later versions of MS-DOS; I'm not sure if these components are so critical that releasing it without them would make it unusable, or if it would just involve a lot of effort on the code side to do so (and then on the audit side to make sure it was really done[0]). Those companies may not exist any longer to even get the appropriate permission from ... though they have a way of popping out of the woodwork when there's a large corporation that could be sued.

Even if memory serves me wrong, and these later versions of MS-DOS are 100% Microsoft code[1], owned free-and-clear without copyright or patent licensing agreements that included any provisions about publication/re-licensing of source code. You then have the legal team.

Copyright is hard, and while I'd be surprised if Microsoft didn't have a legal team that understood software licensing and all of the fun and turmoil of Copyright (multiplied by every country they operate in), I wouldn't be terribly surprised if the idea of Copyright and 'All Rights Reserved' are indistinguishable to -- at least -- some of these lawyers. However, assuming even that's not a problem, getting everyone in a room to clean all of the guts off of the floor is both a time-consuming and not-at-all-easy task. I've done the 'large multi-national corporation thing'[2] and IP was something you didn't pull an "it's easier to ask for forgiveness..." thing on. I created a security product, software, hardware and all for the company, with direct assistance with Microsoft. My boss made a handi-cam video which Microsoft showed at the MVP summit. I spent weeks trying to get permission to upload the video to YouTube or have it put on our marketing page (made little sense there; but probably the easier path). I didn't give up -- I was told in no uncertain terms that I was forbidden to upload the video and that I should delete all copies of it (which I did) with various IP-related reasons given[3].

Honestly, I'm surprised they bothered with MS-DOS versions of that age. It's really neat, don't get me wrong. I'm guessing they either believe that the historical value, good will generated (marketing), and such is high enough to warrant the effort/time/cost and/or the effort was minimal due to it being a simpler time, smaller team, era of software development[4].

[0] And even then, just because you're right, doesn't mean you won't be sued; nor that said suit won't cost more to win than it would ever possibly be worth.

[1] Honestly don't recall -- I can only remember things I saw, vaguely, from my old PC and while I can't remember specifically seeing a notice about third-parties in the copyright notice that appeared prior to the prompt display, I thought a few of the things it shipped with (very possibly having nothing to do with DOS) having been part of the installed product.

[2] 17,000 employees at its peak, globally (plus many, many contractor/temp), so not 'massive' like the likes of Microsoft, but big enough that several bureaucracy-dances were required for some of the most basic-sounding things when it came to software licensing (and a whole lot of other things). We were in tech, but not software, so there's the artificial barrier of explaining to a lawyer the idea of a GPL-style license. Fun times.

[3] I knew our IP policy and worked in security the year prior, so I knew they weren't screwing around. I also assumed nobody would have the desire (or stones) to upload it. I discovered at a job interview that went really well that the reason I wasn't asked a large number of technical questions was because they found the video of the product I had described on my resume and it demonstrated everything they were looking for. I'm not sure who did it (it wasn't me -- I signed the contract, so I really did delete all of my copies), or when -- years (and one merger) later they were far more lax, too.

[4] And I'm really hopeful this is the God's honest "everything but the license" original code, warts and all, because it'd be fascinating to look at something produced at this time under the pressures that were all around them. I remember seeing videos of Bill Gates where he looks like he's going to outright vomit Microsoft blue -- unshaven, unshowered, and talking to a room full of people and a camera.