Again, you're looking at it wrong. Or weird, at least. It's like asking why I'd be mad that the brand of cookies a member of my family eats gets a price hike if I don't like them myself. They're still in my shopping cart every week.
I don't have a concept of a "primary sub holder". It's stuff a group of people gets for the group, and who is paying for which specific parts of the fixed expenses is lost to the mists of time.
I get that US and anglo cultures in general are less collectivist, but this seems more extreme than that. Surely the concept of a close-knit group of people sharing costs without much precise bookkeeping is not completely alien to you. Do you split grocery shopping with the rest of your household? I mean, I did that when I was sharing an apartment during university, so maybe it's an age thing?