At the end of the day, it boils down to a choice as to how you setlle the intellectual property rights which derive from your creative work. Either you restrict the use, sharing and altering of your work by others; or you give a limitless license and anyone is free to do whatever.
Putting your code out there for free redefines the relationship between you and the users of your code. And it does so at both sides at the same time. It's the latter part which usually tends to be forgotten.
As a developer, sharing your work under an open source license is not an obligation to provide premium support. Or any a support at all, for that matter. All you did was share your work, others are free to either pick it up and use it, or walk past and choose something else.
As a user, an open source license doesn't come with a warranty, nor does it imply that you are entitled to support from the maintainer. All you get from an open source license is the freedom to use, adapt and share the software under a similar license.
It means that maintainers of open source projects are well within their rights to say "No. I'm not going to do that." to whatever request you submit to them. Whether that's an issue queue or an e-mail.
When a maintainer receives a request to add a complex feature or fix a complex bug and the request starts with "We use your code at our business / company / organization..." then it's perfectly acceptable to reply with "I would love to. Please contact me at my e-mail address and I'll send you my offer." As a maintainer, open source still leaves you free to charge for any support people solicit with you. You're even free to differentiate between "this is what I'm willing to do in my spare time, this is what I'm willing to do for a price."
The big question, then, is: Why don't open source maintainers do this?
That's where the answer gets complex. For instance, the jump from side project to sustainable income is difficult. Not all projects would yield enough support / servicing work. And not all projects are interesting enough to turn into a full time job. Depending where you live, making a little extra income from a side project may even get taxed away, making it just not interesting to turn it into a side business.
So, why don't maintainers then just refuse largish questions if they can only work on a project in their spare time? I suppose that's the hard part: publish creative work means putting a part of yourself out there. When facing an audience, being able to take a step back from criticism and feedback is probably the hardest part to creative work. There's all sorts of rationalizations one makes to defend a perceived requirement or obligation to keep working on open source projects, even when it's to one's own detriment. Learning how to deal with that in a healthy manner takes time and experience.
First time I've heard that on HN ever. This needs to be common knowledge so more people realize how valuable they really are.