I run an aviation business. Sometimes its the people who offer to pay for assistance without being asked are the ones who receive it for free - they have recognised that it is how I earn my living and that what I have done for them has value.
Exactly the same for me. Every working day I'll probably spend as much as an hour on the phone/email with no charge to the person inquiring. In the long term, it comes around again, so I'm happy to do it. Nearly no one would have the nerve to ask me to provide my service no charge.
Though a few years back, a kindly old gentleman who owned a CHIPMUNK with a traumatized GYPSY MAJOR got two days of my service in exchange for nothing more than a ride (which I did not get before his passing). He just seemed the type of person who had worked his whole life in aviation, and serving Canada in the military, so it was time for him to feel appreciated. But he had offered to pay....