Why did civil rights activists decide to focus their desegregation efforts on interstate buses?
One of the problems with a system of legal constitutionalism is that the courts have no means to enforce their rulings. In the case of Boynton v. Virginia (1960), the Supreme Court declared segregation on interstate bus and rail travel unconstitutional. Yet despite this, Southern states willfully ignored the Court's ruling and so the Jim Crow laws remained in force.
The civil rights movement fixed upon the issue as a useful means of determining how far the authorities were prepared to go to ensure that Boynton was properly enforced. Interstate travel was an especially important issue on which to focus because it would be more likely to gain nationwide publicity, bringing the civil rights cause to a much bigger audience.
Buses and trains would inevitably travel outside the boundaries of the Southern states and so failing to ensure adequate enforcement of the law would mean that segregation was effectively being exported to parts of the country where it was considered completely unacceptable. This made it a matter for the Federal government, not just for individual states. The civil rights movement would then be able to have a much better chance of success as the Federal government would be much more amenable to pressure and influence; whereas the Southern states would continue to ignore civil rights campaigners and their concerns with total impunity.
The “Freedom Riders” chose to target interstate buses for desegregation for two main reasons.
First, segregation on such buses was clearly illegal. The Supreme Court had ruled in Boynton v. Virginia that segregation in interstate transportation was unconstitutional. This was true of the buses and even of the bus stations in the various places where the buses stopped. Therefore, the activists knew that they clearly had the law on their side.
Second, interstate commerce is in the jurisdiction of the federal government. Things like the lunch counters where sit-ins had occurred were under local jurisdiction. But the buses were under federal jurisdiction meaning that incidents on the buses would force the hand of the federal government. The activists wanted to make sure the federal government had to get involved.