It's not their fault, but it is their problem (if they want a larger playerbase). Windows 7 is, to my knowledge, still a fairly popular OS, so it's a good idea to support it.
If someone reports a problem, don't just tell them "it's not their fault that you are doing/using X". (I will admit "dose not work with windows 7 " is not very helpful, but still)
EDIT: To clarify, they don't have to add support for windows 7. I just don't think your reply was very helpful.