That's a fully general argument against strikes. We shouldn't punish Piggly-Wiggly's customers by striking; they might not be able to get their groceries somewhere else. We shouldn't punish drivers by striking at the auto shop; some people won't be able to get their cars repaired.
The big difference is that r/StarTrek is nowhere near as important as a grocery store or auto shop. People need to eat to live. People need to get places. People don't need to discuss Star Trek online at all. Much less do they need to discuss it specifically on Reddit. So the argument is more like: we shouldn't punish customers of Sam's Nail Salon by striking; they might have to go to Pat's Nail Salon a couple blocks over instead, and that's just not fair.
In point of fact, preventing people from entering a facility is how picketing works.
They are making moderation more expensive and more difficult. This is a labor issue. The fact that the workers are unpaid doesn't change that. The bosses screwing over workers is not unimportant.
Same thing as if a movie theater were engaging in wage theft against their employees: wage theft is important, but you seeing a movie at that particular theater isn't.