Free software has never been about demanding corporations open source their intellectual property. For example, Stallman didn't bring a bunch of protesters to Digital Equipment Corporation and the Bell System to beg that they relicense PDP and UNIX as GPL. What Stallman did was create an entirely new operating system that is not UNIX which let freedom loving people use UNIX while escaping the restrictions that were imposed upon users of UNIX.
Richard Stallman wrote at length in the past about how he feels it's ethical to use non-free systems to build free systems if there's no viable alternative. But you can only do that if there's a clean division between what you're doing and what the hardware vendors are doing. Unfortunately it's messy in the embedded world. These makers don't abstract the products they build like Intel does. They rely on legal means instead to secure their advantage. While many corporations might view an agreement to access those bits under restrictive terms as a good thing, it can lead an open source dev to feel like the proprietary stuff, which they intend to decouple themselves from and ultimately escape, is instead being rubbed in their faces. No one wants to be constantly reminded of all the freedom they don't have.
So in other words, it's just a compromise. I'm sure if they could find someone willing to manufacture a truly libre phone, they would have used them instead. I think the FSF has a good understanding of the open source developer's needs / wants / desires and this compromise is perfectly in keeping with that. Perhaps one day they'll attain the obvious end game of a libre phone, which would be a ham radio that looks like iphone with unfettered access to ss7. It will be anarchy. https://youtu.be/eXnvTwRBrgc