“...enforcing the law on an interstate freeway is the responsibility of the local state”
IMHO, states which refuse to keep protests off interstate highways which impedes / stops traffic should lose federal highway funding.
unless the governor or mayor restricted the highway patrol from arresting or confronting the freeway protestors, in which case the governor or the mayor might assume legal responsibility for any outcomes (eg negligence).
jury selection for a civil lawsuit if one is ever filed in a local court could be a very interesting exercise.