It shouldn't ever be the leader's responsibility. I won't go into reason why because the people who have already commented have done a wonderful job doing that already. I don't want to be repetitive, it's unnecessary.
Though, I'd like to add a little bit to my opinion. I think a leader can sometimes, on rare occasions, be responsible or partly responsible. For example, if they set a bad example by doing that same thing and then try to punish someone in the group for doing something they've done themselves, they would be unfair and just be going against their own word. As a leader, you do have a responsibility to set an example. You act the way you want others to act. A leader shouldn't expect their group to be nice and follow the rules all the time, if the leader breaks their own rules or misbehaves.
In your case, I feel you aren't responsible. You are setting a good example and doing what you can. People can just sometimes have a bad day.