I don't see how publicly shaming someone (and yes, this is how I interpret the intend of your question) for the act of thanking the author of a project is going to help anyone.

It is offered free of charge, so why should it be despicable to use it free of charge? Maybe they do actually donate to the project, contribute code, or support in other means.

For example this very post where they thank the author is probably a source of motivation and acknowledgement that might have a positive impact on the project. They could have refrained from doing this but instead they took the time to write a very enthusiastic comment.