I see this more as a lesson to people who judge others for how they choose to spend their resources. Surprised? Yeah, me too, but it's the way that makes the most sense.
That's not how I'd interpret it. Jesus doesn't tell the apostles to mind their own business because her resources are hers to dispose of, he tells them to mind their own business because he likes that she decided to dispose of the resources on him. It seems clear that he's putting his own momentary pleasure above feeding the poor.