The “request membership” link appears because users are allowed to request membership in a group that would grant them access to the topic. This doesn’t work for paid groups which people cannot self-register in Discourse for.
I think we should improve the enhanced 404 here a bit to indicate you need membership to a group @riking, even if the group doesn’t allow self-registration? Perhaps you could take that as a low-priority todo?
I’m going to guess that’s the difference (it is definitely true about my setup). I opted to go with the plugin the advertise activity in private category plugin for now, better suits my needs.