The problem is they _are_ involved. Epic is suing Apple, and parts of their arguments are that Apple supposedly has a monopoly, and that Apple is supposedly overcharging. If Apple can say that Valve charges the same or similar to Apple, then the overcharging argument can go away. It's quite unfortunate for Valve, and they haven't done anything wrong, but quite legal as well.
It's like there's a traffic accident near your shop and you get a subpoena for all your security footage. It's unfortunate for you, it costs you money when you have done nothing wrong, but you still have to supply the data.