The reason we should be paying for foster kids' college is because the state is their parent, so it's our responsibility. In a country that wasn't shit, regular people would be jealous of how kids who were wards of the state lived, and how well-raised they were. There's no clearer illustration of our values than the fact that children who, through no fault of their own, have become the responsibility of the state are treated like unwanted trash. The idea that a society like that could figure out how to ethically treat prisoners or immigrants is laughable.
Yet these are children, specifically, who deserve every opportunity we can afford them by default. Not “hopeless addicts” or some other group deemed not worth saving by so many of us, but people quite literally the epitome of worth saving. These people need every ounce of reassurance that we care and that they can integrate and function in society. That they deserve opportunity as anyone else does.
If we had to be self serving we could look at it like “each one of these people is statistically far more likely to be a burden on my own children in the future, so a small investment now could save a lot later”, but we seem to fail even in being selfish about it. I find this topic heart breaking.