This is a fantastic example, from both perspectives.
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.
> 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)
In the case I'm talking about, the developer uses the BSD licensed library and contributes back to it because they do not want to maintain a bunch of diffs and because they're a developer who gets open source.
Sure, the overall program is closed, but the existence or not of one small part of it was never going to fundamentally alter that.
Most likely these decisions are not to be made by the developer but rather the employer who actually own the copyright of the code (assuming they're a standard employee). The question from here should actually be:
Can we rely on a proprietary software vendor to push even minor fixes upstream? From the companies perspective, this is giving away the developers time to competitors for free, basically paying for every competitor to be an a level playing field with you. A companies responsibility is to appease shareholders by making money, not to make good software or help software ecosystems.
I'd say that most likely, yes, developers would like to push all their changes back upstream. Also I think that companies/managers not understanding of OSS would definitely NOT want to.
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.