FWIW, after looking through their headers for a while I would say that
1) they are close to staying within the things provided by make headers_install; if anything, they tend to strip more definitions.
2) the chief difference is that they pick some garbage from files that lost #ifdef __KERNEL__ when they had been excluded wholesale by */Kbuild mechanism. What they pick is blatantly useless for anything that might use libc and ought to be removed on technical grounds - it's a pure namespace pollution for everything that includes these headers (see below)
3) potential for userland disruption from such removal is very low - AFAICS, their headers do not include those and neither do the kernel headers they do need. In other words, the program would have to explicitly ask for including one of those files. While that is not impossible, resulting breakage will be very visible and the fix will be "so remove that #include, dummy".
4) for a long time _all_ libc implementations had been encouraged to take copies of kernel headers and strip out the kernel-only parts. make headers_install is a technically superior solution that appeared circa 2006. When did development of Bionic libc begin, again? Mid-2000s? If so, they simply followed the common practice for that time.
5) It was *NEVER* intended to force e.g. glibc into GPLv2. Even before 2006. Really. If somebody wants to try and claim in court that their contribution to the kernel had been made with intention to put every distribution out there into copyright violation, I would expect the court taking very dim view of their actions. As for the reaction of kernel *and* userland developers... <shudder>
6) Claim by Bionic libc developers they just followed the common practice, directly sanctioned by Linus and followed by every distribution out there would probably be very hard to reject.
7) (1)--(3) is based the impression from looking through their headers. I have not done an exhaustive analysis of all that pile - I have better uses for my time and what I have seen makes it very unlikely that anything in there would be non-trivial to fix or that switch to make headers_install output as fodder for their script would cause breakage for code using their libc. I agree with bkuhn - the odds are very low.