You shouldn't let that type of person get to you :)  There is always one...ok, two ... in a crowd.

There are way more people who are appreciative of freebies, even if they do not offer commercial use privilages.  Unfortunately it's the few that jump up and down and cause a scene that are noticed and remembered.  The appreciative ones are usually quiet and silently thankful. And there are way more of those! :)

Next time you get a message like that, delete it. Don't even bother to reply because that is only feeding their cause and giving them more fuel for their fire.

Life is way too short to get your back up over a few vocal people. 

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi