Thresh's ulti doesn't make Sens
his ulti is called "The box", it means that you trap someone into something he can't escape, in this case if you are trapped by thresh and try to go out you take a lot of damage + massive slow.
The thing is thresh's ulti range is too short for counterplay and only used if you grabbed someone. And that's why a lot of people use it to disengage by pressing R and the enemies can't follow you. I think this should be removed by activating the walls only if the enemies go through it while being inside the box, thresh already does a lot of things, his disengage should be harder to do.