If someone is CCed in an email you receive, and you respond to that email, it’s common knowledge that you’re supposed to reply all to include the CCed person, right? This is a known thing.
I'd second this. If you're the sort of person who likes mail written to you, rather than the often tangential involvement in receiving a carbon copy of mail to someone else, then you may try to spare others from receiving such carbon copies themselves. Or you might have no strong feelings, and as was already mentioned, your email client's reply button default is just repy-to-sender-only.