In short, it's a matter of attitude. No, you're not obligated to respond to bug reports, but bug reports do provide value to your project, and not acknowledgin

In short, it's a matter of attitude. No, you're not obligated to respond to bug reports, but bug reports do provide value to your project, and not acknowledging that value is just as rude as their expectation that you fix it. The correct way to address this is an auto responder that says "Thank you for the bug report - we appreciate it; however, as this is a volunteer project, we do not guarantee a response, timely or otherwise. If you have a commercial, pressing need for this bug to be solved, we suggest filing a paid support ticket here: <link>." An important point is that reviewing a PR is also a time-consuming effort, the only difference is that it requires time on the submitter's part as well; I would be wary of recommending submitting a PR without talking to me first, since I may not have time to review it either, and then you've just wasted your time.