Why do people think they can dictate my every moment

I have a situation at work where some people feel that their private lives take precedence over everyone else's. Admittedly, I contributed to that environment by trying to be helpful to subordinates in exchange for their help later. But when I cash in my "I scratched your back points" I get "Psyche!" So I have had my weeks planned for weeks. I know when I will have days off and I plan accordingly. I also know to plan something for late in the day because I can't count on these people.
So one of the Russian girls wants her birthday off. She didn't say anything until the last minute. It is today. She also wants the weekend off. Now, I've known what my birthday was since I was five. I also have known since I was 17 that if I want time off, I have to ask for it a couple weeks in advance. My manager thinks that I can just come in for her on my day off. Despite the fact that she has left work without telling anyone twice, that one of those days she did not finish her work and that she has used guest phones, essentially stolen guest food from the breakfast bar and frequently takes extra breaks. And she should still have special treatment? I don't think so.
And I told him the odds that I would come in for her were slim to none. I have a standing appointment on Wednesdays with a therapist/spiritual advisor. I've gone three weeks without said appointment because of these people and every one's crazy needs. If I am to resist going all Chernobyl on them then I am not missing my appointment today.
And it isn't fair. No one wants to jump in and do anything extra when I am short handed. I have always bent over backwards for my employers and my family only to be shafted when I need something. My manager needs to say no.
We need to not be forced to sign guaranteed contracts with foreign nationals who come here to work. It is a privilege to work here. It is a gift. And it is being abused. We cannot fire them for the same reason's that we regular Joes and Janes can get fired. We have to place them in other jobs if we let them go. And she is walking all over everyone. She is even abusing her friendship with the other girl who works with her. No one tells her no. Everyone bows to her demands like she is the lost Romanov princess. I guess I am a Bolshevik at heart.
And you have no idea how largely I am grinning right now.