The injuries are possible in WO games in order to keep the playing field as equal as possible. The idea is not to punish the team that wants to play (they get training minutes) and not to favour them either (they may get injuries). It can be argued that they are punished anyway as they will cannot get 48 minutes in a single position and so on, but this is anyway the reasoning to make the best out of a bad situation.
Of course, there are other ways to allocate injuries in WO games or other ways to reach a meaningful balance that would simultaneously reduce the server load. I just think the BB team has their hands full of work, and the current system is known to work so it is a low priority to change it. One thing to consider about the server load is that the WO games are probably not a bottleneck or in the worst path of the system complexity. There is a certain number of games every week, and an unknown number of them will be WO games. Thus, there needs to be capacity to run the full number of games each week. But future improvements in this area are anyway possible.