Yes, j/s is free for commuters. IIRC, j/s is your *only* option on probation. Maybe it's changed, I don't know. I used to commute SFO-MIA back in the day, and you were SOL if you couldn't get the j/s, even if there were empty seats in back.

(usually you'd have a good captain and/or nice gate agents who'd get you on anyway, but it wasn't something you could contractually count on)