…But I still find it a bit weird that I am able to select my opponent’s casualties…
That’s the way it is intended to be in PBF games. So nothing to be worried about.
Imagine that you needed to wait for your opponent’s decision after every dice roll…
That would pretty much contradict the aim of PBF: to allow for asynchronous play.
Of course you have to choose the casualties reasonably.
In most battles there will be no conflicts about how to choose casualties reasonably - as it is often obvious.
There are of course battles where the selection of casualties is not that obivous…
In those cases it is standard to ask the opponent for his Order Of Loss (OOL).
So take this as a rule of thumb: “Always choose the casualties reasonably, choose them for your opponent the same way that you would like your opponent to choose for you. In case of doubt - ask your opponent.”