Why can a college charge higher tuition rates for rich kids vs poor or middle class kids? Isn't this discrimination based on family income? A rich person pays the same amount as a poor person when it comes to purchasing anything but that is not the case when it comes to college tuition.
In the US at least, discrimination is legal (and sometimes even required) unless it is discrimination against specific, protected characteristics, and even then it is sometimes allowable if it is "necessary".
Income, whether of an individual or that individual's family, is not in any list of protected characteristics. As such, discrimination upon it is fully legal.
Technically, they charge a lower rate to certain means-qualified students, by offering assistance to those with an income below some number. This is legal because it hasn't been made illegal. In general, anyone if free to give assistance to someone on the basis of income. The closest you can get, in the US, to laws prohibiting discrimination based on income is some states have law against discrimination based on source (not level) of income, e.g. RCW 59.18.255.