So presumably we're not talking about adherence to the historical facts

No, we're not. Take "Spencer the Rover" - presumably it's not a factual account (there aren't any mountains near Rotherham), but that doesn't justify changing the song. If anything, Bert Lloyd could argue that the historical facts were on his side - I'm sure there were colliers recruited to the Napoleonic Wars, and why should the ploughboys get all the songs? For me, at least, it's not about saying "this is how it was" so much as "this is how the song was" - and getting that right (or at least doing my best not to get it wrong) is actually more important than getting the history right.