The Founding Fathers knew, and accepted, that they were engaged in treason and rebellion against the British Crown. They made no pretense that what they were doing was legal under British law. Their rebellion was based on man's natural right to overthrow tyrants. However, the Founding Fathers knew what their fate would have been if they'd lost their rebellion- a short rope and a long drop.
In comparison, the Southern leadership made the ridiculous claim that their rebellion was somehow legal under the Constitution. When they lost, they were treated with much more restraint than probably any losing side of a rebellion in history, certainly with more restraint and forgiveness than King George would have shown to the Founding Fathers.
The southern patriots WERE legally allowed to seceed according to the then US Constitution: Ever heard of the 10th Amendment: List me where it specifically stated in the Constitution that a state couldn’t freely leave the union?
Tell me exactly where in the U.S. Constitution that secession is expressly forbidden.
No where.
In fact, this is the precise reason that the U.S. government had to release Jefferson Davis from prison in 1868 without a trial since it was afraid that, legally speaking, there was a very good chance that the courts would rule that secession was indeed legal, as Davis was going argue in his defense.
To this day, no federal court has ever ruled on whether or not secession is legal under the U.S. Constitution.