You might have to check with a lawyer to see if it is legal. Many preschools are privately run and they can make up their own rules. However, if the school is taking state or federal dollars, they might have certain rules they need to follow. If they have grant money that says it is only to benefit specific socioeconomic groups, then the family income will plan a part in who is accepted into the program.
Unfortunately it's really so, up to 10% of a Head Start program can be made up of children from the families with over income but if the program is full without them, the latter can not be enrolled. Only Oklahoma and several more states allow qualification for a wider category of children, in other states, there are a lot of american families with incomes too high to qualify for Head Start. You may consider some other pre-K opportunities for your child, and if you possibly need extra means for enrollment into some private establishment, look at this site