To strictly play devil's advocate for a moment; why do developers think it is appropriate to benefit from the free/open source library, yet not provide that same benefit back to others (in the form of using it in the proprietary software)? To me, this was the exact type of situation in which the GPL was crafted to avoid, and part of why rms didn't just declare his software to be in the public domain. I'd also imagine rms' response to this complaint would be along the lines of, informing the developer in question to work on a free, non-proprietary version of the software, so they could in fact, use the library (totally disregarding the fact it is due to external pressures, such as the employer, that they're developing proprietary software in the first place).
This is, of course, assuming the original developer licensed the library under GPL on purpose and knowing the resulting implications, as you eluded to.