@druid It's also a trap for developers. With free software, you have users, and some of them may have the ability to help you fix bugs or may contribute code (not all of them, though - in most cases the vast majority of your users will not be coders or developers, and developers shoot themselves in the foot when they don't recognize that. The exception is if your software is a tool primarily used by coders and developers).
However, with paid software you have CUSTOMERS. And CUSTOMERS feel they have the right to demand your attention. After all, they paid their hard-earned money for your product and if it doesn't work the way they expect, then you had damn well better hop to and fix whatever the problem is, or they will leave you terrible reviews and do whatever they can to make you sound incompentent, lazy, or evil. And that will have a negative effect on future sales!
I realize I am generalizing and there are exceptions and outliers that don't fit these patterns, and especially with free software you will still have a few demanding users, but you can ignore them if you choose to because it's not like they can affect your sales. They may, with or without justification, persuade others not to use your software, but that won't take any food off your table or leave you shivering in the cold.