The 'de facto' segregation of the northern states was a shadow of the reality of the legally enforced segregation of the south.
No, it was because white people would refuse to share busses and schools with blacks. The free market responded by giving what most of its customers demanded.
Anyone thinking racism and free markets are mutually exclusive, should read this:
http://www.mtholyoke.edu/courses/sgabriel/racism.htm