I care about the quality and usefulness of my software. I give it away to give back, because I have been given much.
There are bugs in my software which I will never discover on my own. When a user offers me a bug report or a patch, that user is also giving back. The bug report is a form of contribution. It gives me information I wouldn't otherwise have, which I can use to improve my software, which can make it more reliable and useful to others.
Most users who encounter a bug don't take the time to report it. Those who do report bugs contribute their time and effort, and the fruits of their contributions help not only myself, but other users of the software.
This applies not only to bug reports, but to suggestions and feature requests as well, because all of them can help make the software more useful to others.
The only interaction that has negative value is to be asked a question that's already answered in the documentation. But even those can be valuable when they indicate a weakness in the documentation that makes the information hard to find or understand, which can be improved, which helps other users. And even the questions which are trivially answered by the documentation can lead to valuable interactions later.
So to charge a user for the privilege of interacting with me would be like asking the user to pay me for the privilege of helping me, for the privilege of giving back. It would be rude, and it would deny many the privilege of contributing through the means at their disposal. It would almost be like keeping a fellow human being in a kind of moral debt, rejecting their contributions out-of-hand, leaving them obliged, with no means to repay (or to "pay it forward").