Your analogy falls apart because there isn't such a thing as an open source mechanic. I develop open source software and I get tired of things like feature requests that amount to "please rewrite this project to do the exact needs of my project/site."

If it's a reasonable request, I'll try to look at it, otherwise, tough luck unless you want to become a client. If one of my clients came to me and said "I did something and some other thing broke" I wouldn't snap at them, but when it's someone who's getting free support, then it's on them to take part of the responsibility to provide as much information as possible.