First of all, glibc supports proprietary software, which is why they allow proprietary software to link to it. So punishing certain programs for license choice by making subtle (and sometimes unjustifiable) changes to API seems like a highly passive-aggressive approach to their ideology.
Second of all, there MUST be a way to preserve backwards compatibility AND allow for future progress. Remember: all of your open-source programs which exhibit this bug are just as broken as Flash, except that if someone tracks it down in the Free software it can be fixed for future versions. But that doesn't help anyone who already has the software installed. I just can't imagine that the glibc people couldn't come up with ANY approach that works for everyone. Off the top of my head I can think of several; probably there are reasons why they are problematic, but in that bugzilla entry even Linus Torvalds was unable to prove that the glibc approach was warranted at all, let alone warranted for everyone all the time.