I didn't work on that particular system, but I can say from experience that systems on an airplane are a lot more interconnected than you might expect, just from seeing them as a passenger. There probably is a sequence diagram somewhere that says, "Inform pilot of movie progress, reduce intercom volume, dim cabin lights, reset passenger overhead lights, consult schedule to determine selected movie, lower projector screen, play movie." And there are probably similar diagrams for landing, maintenance, encountering problems, and so forth.

Furthermore, the IFE might not need to talk to critical control systems, but they both might need to talk to some of the same other systems. Cabin lights seem like a likely candidate. Displays for the pilot, too. And maybe maintenance fault logs.

It can be a real hassle to have totally isolated systems on different networks. You can do it, but you need a Good Reason. The easy thing, from an engineering perspective, is to make it so everything can talk to everything.