If you're a firefighter, is it labor to be at the station playing cards, just because there aren't any calls coming in right now? If you're an ER physician, is it not labor to be waiting for patients on a quiet night?
> Reading a piece of fiction on my couch is not labor under any reasonable definition, because I am not working.
If it's a Saturday and being on-call is preventing you from buying groceries or going to the movies, then being on your couch reading a piece of fiction is labor. If it's the Fourth of July and being on-call is preventing you from having a beer at the barbecue, then that's labor.
"Labor" isn't just the activities for which you are actively producing value for somebody else. Labor is any time your allowed options are restricted as a result of your employer's decisions. Sometimes, those restrictions dictate only a single option of being on-site working on a specific task. Sometimes, those restrictions allow multiple options have some flexibility to them, but the existence of those restrictions at all means that it is still labor being required of you.