Why is that the question? I think it's clear that some of them work for the president, and the president (perhaps filtered through cabinet members and others) tells them what to do. But that doesn't mean that Congress can't put limits on when and how the president can fire them.

That isn't necessarily the case for all officials. For example, I believe Congress can't prevent the president from firing a member of the cabinet. But that doesn't mean the president can do whatever he wants to anyone.