Many pilots ride the JS to get an extra non revs on.

In his mind he’s is deciding to ride the JS and leave you, or ride the cabin and leave a non rev.

Or he worries about other pilots with better priority in the cabin or JS. If they have separate listing procedures then he can get screwed trying to jockey between the two at the last minute and some just list for the JS and don’t worry about another pilot trying to ride the JS and cancel their JS listing and start over with a cabin listing.