OP literally states they're not much of a gamer, it's very possible they don't know much about gamepass.
And? The first thing they'll see is it's a subscription. You were right to remember them the games were not theirs to own, but I fear that part got lost in the rant.
I you used your brain you'd realize nobody said game pass was free and OP is probably not stupid and checking what game pass was is probably the first thing they did when they heard it mentioned here, if they didn't know beforehand.
Why do you think everyone is stupid? That's not things you should have to warn people about, yes paid subscription services cost money, they're clearly advertised as paid options, it's not a fucking grey line in an insurance contract or anything.