It’s hard to know how to weigh in on this controversy, since the definition of “busing” is itself uncertain; since it’s hard to tell what Joe Biden means to say now, let alone what he meant to say decades ago; and since the criticism of him by his political opponents is so likely to be deceptive as well. But these thoughts:
1) It is unconstitutional and bad policy to assign students to public schools on the basis of their skin color.
2) This means that Jim Crow segregation was unconstitutional and bad policy; it also means that racial balancing of schools (which I have no doubt is now supported to one degree or another by all the Democratic presidential candidates, including both Joe Biden and Kamala Harris) is unconstitutional and bad policy.
3) Some federal judges, federal bureaucrats, and civil-rights groups thought that a good way to end Jim Crow segregation was by affirmatively assigning students to schools on the basis of skin color. In some circumstances — where this was simply a way of ensuring that students went to the schools they should have been going to all along had there been no Jim Crow — this might have made sense, but often it was, to understate the matter considerably, controversial. Some of the criticism might have been rooted in a defense of Jim Crow, but often it was not, because the liberal definition of “segregation” included not only the de jure but also the de facto variety, and because using racial discrimination to end racial discrimination is always problematic.
4) The federal-state and voluntary-mandatory distinctions that are often important in other contexts don’t really have much salience here. That is, it can’t (as a matter of constitutional law or sound public policy) be left to the states to decide whether to engage in racial discrimination; likewise, if a state decides to engage in such discrimination, it’s misleading to call it “voluntary,” since while the state is not being forced to adopt the policy, the resulting discrimination is not voluntary for the students who are then assigned to schools on the basis of their skin color. And this is true, again, whether we’re talking about old-fashioned Jim Crow segregation, or the new and politically correct discrimination undertaken to achieve “diversity.”