I think we don't have the same definition of optional -- like someone else has noted, maybe the better word was "flexible". The way you're using it is the super manipulative "yeah it's optional, but why would you want the rest of your team to suffer?". Does that not sound manipulative to you?
As far as your impact-on-peers argument -- you could "optionally" also stay 5 hours after when you normally go home to help reduce workload for your peers and help them, do you do that? No? What about 4 hours? what about 3? 2? Where is it fair to stop? The rest of the adult world calls this professionalism, and you stop at what's required of you as your job duty, put forth in your employment contract. In the course of fulfilling that duty you're expected to be reasonably courteous, not to subscribe to some weird hostage situation where the rest of your team suffers if you don't do something that was marked as optional.
As far as your impact-on-peers argument -- you could "optionally" also stay 5 hours after when you normally go home to help reduce workload for your peers and help them, do you do that? No? What about 4 hours? what about 3? 2? Where is it fair to stop? The rest of the adult world calls this professionalism, and you stop at what's required of you as your job duty, put forth in your employment contract. In the course of fulfilling that duty you're expected to be reasonably courteous, not to subscribe to some weird hostage situation where the rest of your team suffers if you don't do something that was marked as optional.