Yeah, but it's not like by using it a handicapped person can't, as far as I know there are no limits on the number of copies available. And I'm far from the first person to do it, there are quite a few articles suggesting it, including one from CNET, which is where I found out about it. And honestly, why would MS think that e.g. blind people need a free upgrade more than anyone else? Most people I know of on disability live a hell of a lot better than I do.
Here's my theory: People who pirate Windows (and software in general) fall into 3 categories:
1) Small time pirates, who use pirated software on their personal machines.
2) PC "OEM"s, who manufacture and ship PCs with pirated versions of Windows. This is mostly an issue in China.
3) Businesses who run pirated software - some of this is just laziness, it's easier to create a single image with an activator then to actually properly activate a large number of copies of Windows, but some of it is also IT departments who need to cut their budget without any reduction in software or services.
I think the small time pirates are largely not worth going after. Not only do they not get much of a return on investment (thousands in legal fees over a piece of $200 software), but the RIAA and MPAA have shown how much of an image problem that can lead to. The PC manufacturers and business who are imaging dozens if not hundreds of PCs daily with non-genuine copies of Windows are a larger financial drain, and a much softer target to go after, at least as far as corporate image is concerned. But, and I can't be sure of this, but I think it's largely been the small time pirates who have developed the SLIC BIOS mods, DAZ loader, KMSpico, and all the other methods for pirating Windows, for a few reasons. First, I can't imagine that the fly by night OEMs having the know-how to create those tools, I imagine they're just system builders, and the businesses can't really have a "Hacking" department ("Halt and Catch Fire" is a really good show, BTW). Second, even if either of them did create the hacks, I don't think the final product wouldn't show up on torrent sites. But, since someone did create them and post them online, now every OEM and business can easily snag those tools, load them onto an image, and make every PC they sell or support look like it's genuine.
So, back to the "accessibility" upgrade. The upgrade requires you to download Windows at the time of install, AFAIK there's no offline upgrade. When Windows 10 installs it creates a hardware profile, based on hardware serial numbers, so it's only genuine for *that* machine, even swapping a motherboard like for like requires reactivating. So what Microsoft has done is create a loophole so the hackers with too much time on their hands don't need to create a Windows 10 hack, the "small time pirates" can just hack 7 and run the upgrade. "Normal" people who don't regularly pirate software will still go out and buy it, because it's not exactly the easiest thing to find and it does seem a little "wrong". So now the OEMs and IT departments don't have any "good" publicly available activation tools that they can deploy on an image (there are a few crappy ones out that, according to the reviews, don't work well, hog resources, and may include adware/malware). They can try to run the accessibility upgrade 50 times a day, but I'm sure MS would catch on to that quickly, and probably become targets for lawsuits.
So TLDR: MS is basically keeping the hackers and small time pirates happy so they don't develop activation tools, so the OEMs and businesses will be forced to buy Windows. And honestly, if that theory's correct, I think it's a fucking brilliant plan.