Share play is sticky red tape to play with.
I'm sure due to some legal loopholes it's 100% A-Okay - the reason I say this is because there are clearly instances where Share Play will deactivate at the Developer's discretion - example, watch the opening video for the game again when you make a new Char. You are not allowed to use the Share menu, it's blocked.
If ZOS really cared about people shareplaying arenas and trials and whatnot, they would have blocked the feature for them.
The fun thing is that some games it can lead to even bigger problems - one time a kid on Destiny shareplayed with somebody - and since you can delete you character in-game the guy backed out to the load screen when he had control and deleted all of the kids characters... funny thing is, this is fully possible in ESO as well. I wouldn't share my account to anybody knowing this.
This basically means then, that PVP also becomes useless to try and gain emperorship if that's on you "to-do" list without enlisting the help of people who will play 24 hours round the clock for you why you have a nap...
This is just making the game less and less appealing in my honest opinion...