Because for like 20 years now they all have lots of extra plugs for surrounds and crap. You can see both the grounding and the cover type here in image 9, they're alongside the modded RCA outs that are connected in this photo.
https://systems.audiogon.com/systems/8367#&gid=1&pid=9Technically of course even with the interconnect disconnected the player is connected to the system as long as the power cord is plugged in. That's the way components work with power, neutral comes back to the conditioner, same conditioner everything is plugged into, everything is connected together. Same reason doing something on the panel or anywhere in the house affects the system. So its nowhere near as mysterious as I let on. That was just my usual trigger trolling. Usually catch at least a few bottom feeders, thought I had one but no, your question is legit. Oh well. Its a numbers game.
Thanks for the opportunity to link to my system yet again. Eventually someone is bound to notice hey, that millercarbon guy he has a system, but the guy who posts 5x as often doesn't? ??? Never know. Could happen.