You don't like it, so you want to say it's 'not allowed'... but it is. If YoshiP and the other developers did not want to 'allow' players to leave instances, they would not have the option something you can choose. The only way to leave then would be to be kicked or to DC. It is expected people will want to, or need to, leave an instance. In order to not deal with people jumping in and out repeatedly (it happened back in ARR, esp when they didn't let people group up for the CT raids) they put a cool-down in. Some call it a 'punishment' but to others it's just a timer to find something else to do in-game or IRL.

Harassing people verbally is clearly outlined in the TOS. It can be reported for a reason. I would like to see you report a person for leaving an instance and see where that gets you with the GMs. That should tell you everything you need to know about 'not allowed' or just 'it annoys me, so I'm going to hype it up as ILLEGAL on the forums'.