Basically the assumption that by sharing your work, you become inherently beholden to the wishes of those that use your work.

Some time over the last ten years or so, much of the open source community acquired the belief that they deserve to be treated as a customers despite the fact they don't pay for squat.

What were people doing based on that assumption? The article says contacting the author to ask for a fix over bug trackers, Twitter, etc is still ok in their mind. What else were people doing?

"Ask for a fix" is euphemism what often happens. It's commonplace to receive reports to the tune of:

  * "Can we get an ETA on this? Some of us need to get real work done"
  * "How you can justify leaving out such a basic feature?"
  * "This project is unusable without this feature"
  * "Why waste people's time by releasing software with bugs like this"
  * "This kind of mistake is totally unprofessional"
  * "The developers must not have the skill to fix this"

These sorts of interactions can be more exhausting than outright abuse, because they're designed to bait/provoke the developer rather than just insult them. And to be clear, this isn't particular to this developer. There was a whole study done on this that made the rounds awhile back[1].

[1]: https://www.cs.cmu.edu/afs/cs.cmu.edu/Web/People/ckaestne/pd...

I assume some of the people do not understand either of these steps, and just keep pestering the author after they say no.