The meaning is that scoundrels, when confronted with evidence of their unanswerable crimes, will instead answer a different question: "Do you support some unquestionably virtuous cause?"
This is the reason that that runners of big cons typically give very generously to charities. See Anthony Trollope's "The Way We Live Now".
Oh, I understood it differently. So it means that some scoundrels will use some virtue (in this case patriotism) to hide behind and not that patriotism is a bad thing?
I wonder if this particular 'virtue' has ever led to anything good. There is a parallel in the difference between, say, love (for one's land) and faith on the one hand and patriotism and (organized) religion on the other - the latter ones have invariably been used directly to promote hatred and intolerance in support of some political goals or in justifying criminal acts.
This is the reason that that runners of big cons typically give very generously to charities. See Anthony Trollope's "The Way We Live Now".