I am wrong for being annoyed?

I know this isn't related. But I need to rant for a min. So my son well step son. I've been there for him since he was two. And his mom isn't really in the picture he sees her two times a year and she talks to him only when he is about to leave to go see her. I feel bad for him that his mom really doesn't seem to care. she doesn't do much for him but buy ticket so he can some see her and she makes it well known that it's a burden on her. It annoys me she left and move far from him and wants us to pay for it. And she tell us we should pay for it its not her fault we still live were we do we should move closer to her. Really? . So this year she said she would buy his school clothes she never has before in the passed 6 years hasn't even offered. She told me and I qoute" he is all set I've gotten him every thing he needs clothing, supply's and all that good stuff" . So I'm like cool she finally is stepping up. So he gets home and has next to nothing 3 pants, some shirts,a little box of pencils. I asked him where is the other stuff he said that's all I got. I was like wtf. No shoes, no backpack, no notebooks or anything. So now I have to go out and get this stuff.I don't mind and I don't care about the cost. But she has no other kids and her and her man both have good jobs and she can't help out with him. I don't get it. Why is he such a burden to her? And its always about money with her. I don't get it. How can a parent not care?