Another question to ask those people. When was Lee’s trial for treason?
There was a time when I actually accepted this line of reasoning, but it's garbage and not a position that I changed recently. Slavery was the issue that divided the north and south before secession and the political battles about whether or not the territories being added to America would be slaveholding states or free states preceded the actual battles of the Civil War. I don't expect historical figures to have cosmopolitan viewpoints, but even by the 1850's abolition was a very well debated topic and people were making conscious decisions about whether or not to support the continued enslavement of blacks. Lee knew the southern states were seceeding to preserve slavery. He didn't see that as a deal breaker for his honor.
Lee wasn't an innocent bystander swept up by events, he made a conscious choice to side with rebellion, probably much more to preserve his land holdings in Virginia than his honor. His work on reconciliation after the war was admirable, and his tactics in battle aren't diminished by who he fought, but it doesn't change the decisions he made during.
It seems Lee understood that secession was nothing more than revolution, which means he knew it was traitorous to take up arms against America.
...Secession is nothing but revolution. The framers of our Constitution never exhausted so much labor, wisdom, and forbearance in its formation, and surrounded it with so many guards and securities, if it was intended to be broken by every member of the Confederacy at will. It was intended for per- petual union, so expressed in the preamble, and for the estab- lishment of a government, not a compact, which can only be dissolved by revolution, or the consent of all the people in convention assembled. It is idle to talk of secession. Anarchy would have been established, and not a government, by Wash- ington, Hamilton, Jefferson, Madison, and the other patriots of the Revolution. . . .
Lees letter to his son January 21, 1861