Unless you drive with the windows up, looks is the only real reason to have one anyway, so installing it under the full top is reasonable if you want the look.
That works for me and I agree.
If you already had one when you installed the full top, you can either leave it installed or have to store it someplace.
If you switch back and forth between tops it also makes sense to leave it installed. It will also serve as a lightweight headache rack, keeping items in the back, in the back.
Having seen a coupe top installed, it is a total b**ch of a job to do and may require 3 people to do it. It is not a casual change. Also, the convertible top has been removed, so I doubt there is any intention of removing the coupe top.
The only real reason to remove it - except to avoid comment - would be to ease access to the back from the front.
As 44thSky commented, there really isn't any access to the rear from the front.
I can only conclude that it was left in place for looks. I am not into the car show mindset, never have been, so there are things I just don't understand when it comes to items like this. While this particular Sky has a nice paint job, and the rear of the tulip panel is masked nicely, to me the Windrestrictor is just a bit of overkill. But, if the owner want's it that way, that's how it'll be, and I certainly won't be critical, other than to say, "Huh, I don't understand."