"...all of the books on Scott over the last century..."
Sorry Steve; have read the introductions (where they exist) to most of the ballad collections, also the few accounts of the work of collectors that I could lay my hands on, incuding Scott's own Journal, and could find no hard-and-fast 'motives' for their having carried out the edititing that they did (or were claimed to have done) other than that they were 'prisoners of their own particular period'.
The bravest stab at examining the work and inspiration of the collectors seems to be (haven't had time to read it fully yet) E David Gregory's newishly published 'The Late Victorian Folksong Revival', and that falls short due to lack of contemporary information.
One of the great gaps in our knowedge of folk-songs is, of course any input by the singers/songmakers themselves; nobody seemed to be too interested in what they did/thought, and now it is too late.
We do have fairly modern examples of singers and collectors editing songs in order not to offend their audiences and readers.
I must have listened to MacColl singing 'Browned Off' a hundred times before I heard the verse:

"The medical inspection, it is a bleedin' farce,
They grope around your bollocks and they finger up your arse,
For even a private's privates enjoy no privacy,
You sacrifice all that to save democracy."

Not a folk song of course, but an example of one being censored for the sake of 'the ould decency'.
Source singers have been known to cut their songs so as not to give offence - it took us several efforts to record the last verse of the Co. Clare version of John Barleycorn which tells of the drunkard "pissing me against the wall", because of the singer's habit of deliberately mumbling the last line.
Elderly Clare singer Tom Lenihan always cut out a 'sexist' verse of one of his most beautiful songs, 'Cailín Deas Crúit na mBó' (Pretty Girl Milking Her Cow) when he sang it for us, for fear of giving offence to Pat.

An old maid is like an old almanac,
Quite useless when once out of date.
If her ware isn't sold in the morning
At noon it must fall to low rate.
Oh, the fragrence of May is all over
The rose leaves its beauty, you know.
All bloom is consumed in October,
Sweet cailín deas crúit na mbó.

Sorry for being a bit long-winded, but I am still convinced that folk song is very much a foreign country with strange customs that we don't fully understand, and probably never will.
Jim Carroll