I signed WHAT?!?!

OK, granted this one was my fault, but I vow to learn from it, and not complain except for about how backhanded and sneaky it seems from this end...

I went in today to work, and check my schedule for this week, and found I was scheduled for work this holiday Monday. I'm not available to work Mondays and thus, had made plans with friends and family, so a conversation with my immediate superiors went something like this.

Me: "Um, I'm scheduled for work Monday, but I'm not even available to work Mondays and have plans."
Associate Service Supervisor: "Oh, your not available Monday?"
Me: "Well, no, not even normally, but plans have been made."
ASS: "Ok, no problem we'll take you off the schedule."
Me: "Ok then. "o_O' *surprised, but ok*

Now, this was weird, and the first time it had ever happened to me in all the 5 years I've worked here, so I went in search of a more knowledgeable employee...

Me: "I'm scheduled for work Monday, but I'm not even available to work Mondays and have plans, do you know why this happened?"
Slightly More Informed Labor Employee: "Oh yea, like a gazillion years ago when you were hired and signed 7 billion forms, on of them was for which holidays your available to work."
Me: "Ok, but since it's not my usually work day wouldn't it be nice if I was told I would be working?"
SMILE: "Yea, no, they dont have to tell you, or ask you or anything, and no one ever remembers the paper they signed either."
Me: "Not good tho, people make plans, have prior commitments, this is a bad system."
SMILE: "Yea, its kind of a gray area..."
Me: o_O'

Oh kaaaaaaaaaayyyyy,
my bad,
still,
this is a really bad fuck you in the ass kind of system.