"to sing songs that aren't from one's own region/area/district/country/whatever"
Nobody said it was "wrong" - that's one of the urban legends.
People running clubs can adopt any policy they wish to present any kind of music they wish - MacColl's clubs chose to promote traditional and traditionally based clubs in order to open up the British repertoire - those who didn't agree with that policy were free to go somewhere else.
As Peggy said - it was a club plicy adopted by Ewan's clubs.
The end result was that the British repertoire became the norm (more or less) - I'll drink to that.
Jim Carroll