Some colleges only consider the custodial parent (e.g. Chicago, Vanderbilt; Princeton if the custodial parent is remarried).
Really, this seems like a policy decision on the part of many of the colleges, since it is a convenient way to screen out close to half of those with financial aid need (about half of kids will see parental divorce, and probably most are nasty enough to ensure non-cooperation with financial aid forms or paying for college) in order to keep the financial aid budget in check while still claiming to “meet full need”.