Ok so that requires a bit of history. There's this company that makes video recorders (for your TV) called TiVo; it's not an abbreviation for anything, that's just the name.

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Anyway, TiVo wanted to get onto the GPLv2 train like a lot of companies, but they also wanted people owning their devices to be unable to partially modify their firmware even as they distributed their copies of the firmware onto TiVo boxes. They enforced this by having a digital signature check that only allowed TiVos own software to run if the software stack was entirely TiVo. It would also work if the stack was entirely FOSS because only TiVo checked it.

For the sake of clarity; Bradley Kuhn of the SFC iirc investigated the TiVo boxes at the time, the signature checks only were an issue if you partially replaced TiVos software, not if you went the whole hog on replacing it; excercising GPLv2 rights was completely possible and you could turn a TiVo box into a XBMC (these days that's Kodi) box. You just couldn't run TiVos software ever again if you did that thanks to a hardware check.

The FSF took personal offense to this practice and dubbed it "Tivoization", which is a shorthand for "attempting to restrict the ability to install or link with Free Software via hardware DRM". (Which yes, is a modification of what TiVo actually did but really that part is just par for the course with the FSF.)

As a result, the GPLv3 includes an explicit clause that if your software is distributed in a non-code form, that you also must distribute all the information required to be able to modify that distributed software, if there's parts on the device that prevent you from doing so. (Or in plain terms: on restricted hardware, you must give up the signing keys if you preload the hardware with FOSS software.)

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This clause is usually just called the anti-Tivo(ization) clause and it's... pretty damn controversial. It's the main reason why the Kernel is still GPLv2 and not GPLv3; Linus Torvalds personally considers this clause to be a significant enough alteration of the "deal" that FOSS provides for the kernel, so he didn't upgrade (which to be clear would've also been very difficult since the kernel is 2.0-only, not or-later, so he'd require approval from all significant contributors at that point). It's also often cited as the main reason why the GPLv2 has/had strong corporate backing but software licensed under the GPLv3 has always kinda had issues with that; the v2 was seen as more "fair" in that companies were willing to work with its terms, while the v3 was seen as basically forcing them to give up important parts of their trade secrets and made them antsy of working with the FSF in general.

And as mentioned before, in some fields (medical is the one I know of, but I'm pretty sure there's a few others), regulatory compliance is impossible with v3 while very much possible with v2 because of this clause.

Thanks for the detailed explanation and ugh, it is so frustrating. I get and share the goal of being able to fully control your devices - but you have to work with what you have. And I think this fanatism and uncompromising attitude is really not helpful in getting there.