I have used WiringPi along my side of work and I am thankful to the author for creating and maintaining it, but honestly I don't understand the motivation behind offering a free product with email support.

There are many different people on the Internet with different cultural backgrounds, experience levels and work ethic and unfortunately it's not realistic to expect each of your users to share the same values as you do. Anecdotally, charging a license fee for your library/tool is a very good way to filter out 90% of the unprofessional behavior.

Also if you rely on your free product to promote your consulting business, I would highly recommend creating a community forum where users can advice each other, and maybe offering paid support on top of that. Also using an open-source helpdesk app to semi-automatically send canned replies to commonly repeating inquiries saves a ton of time and keeps the assholes off your back, as long as the canned replies are reasonable and polite.