Some want to do optionals for more chests and XP. It takes longer to complete. Some optional chests have a chance for named loot, guild renown bonus etc.
I believe it's the guy with the star who should set the rules. If you want to zerg you should at least ask up front if you're not the leader. If you are the leader you should post it in the LFM so people who join know. If no rules are set in the LFM then the default should be a bit slower than zerging. I expect a group to want to do optionals if no rules are posted even if the quest is elite. When I lead groups I ask if people want to do the optionals and 70-80% of the time people actually say YES and not no. My impression is that most players want to get all the chests in the quest and conquest, ransack if possible.
I almost only run elite quests up to the Amrath quests. My toons are TR's so I like to get the extra XP and favor. I like to have a normal pacing of the progress instead of the zerging where you have to rush as fast as you can to each chest. Tempest spine is a good example. The inside map is not working and you just have to ensure you keep pace with the pack to not get lost or lose any chests. It's rather stressful.
I understand some people like to max the xp / minute by running through quests as fast as possible. So it's fine if they post LFM's with fast runs, byoh etc. Then you know what to expect. But there are actually players around who like a slower pace who are quite capable.