When you push an animation in nla editor you can not edit it until you make it green. if it’s not, you can only stretch it. Move it cut it… as in a video editor.
The edit mode refereed to is in the NLA. You can also get to it via its menu. (and I just learned you can also Shift-tab to edit stashed action.) You asked “Why” to me this it a great help and I would go and open the action in the action editor. And often got quite confused. The edit mode in NLA clears up a lot of issues I had with editing actions. I hope you solve yours.