How is this NOT harrasment/griefing?

[Arkine]

Because one person has the same right to play the game just like the other 7?

I don't quite understand how that means that said one person has the right to ruin the game for the other 7 but alright.

[Dragon]

In this case the few have the power and those with power don't really give a crap unless they are the courteous ones. SE can't/won't do anything because of the achievements set in place so tough luck unless they either get rid of those achievements or make behest an instance like Ifrit. Seriously, people only do it for achievements which is a goal, just like leveling up.

The only problem in the OP's scenario (and that's all it is now - a scenario - we don't know if he's lying or telling the truth) is that the Lv.50 player character didn't just kill off the enemy boss during the Behest which caused the others to lose their bonus EXP. Nothing else, while you can deem as immoral, was in itself objectively wrong.

You dont even kknow the other side of story, none of us know. It could be OP exxegarting stuff to jjustifies his reason when he probably didnt even bother to send tells to the lv50 guy. People have known to twist stories to favors them, i personally do not believe anyone would kill normal mob and leave the NM alone, i could believe he kills everything for aachievement purpose, but leaving NM alone is probably the story OP made to get us to side him.

Can't say I believe it, but I'm agreeing with this post.

people with capped class won't able to do achievement then

A cap in an FF MMO does not mean someone higher than Lv. Y can join in, but that Lv. X wherein X>Y will be synced down to Lv. Y to access said content.

Capping an event means your level is reduced to fit that cap if you join that event.

You get to do your achievement and the others get their exp.

Happiness for everybody.

Exactly.

Because one person has the same right to play the game just like the other 7?

If the one player is in fact harassing the other seven by doing what I described earlier in this post, then he's not simply playing the game at this point.

[Colino]

I don't quite understand how that means that said one person has the right to ruin the game for the other 7 but alright.

Well, it's SE who added those achievements in the first place. If you are a completionist you'll want to do them regardless of the level.