It does upset me when this sort of “dog in the manger” rubbish raises its ugly head here from time to time. My wife’s grandfather served in the King’s Own Scottish Borderers at Gallipoli , the Somme and elsewhere in the 1914-18 war and wore a kilt through all his time in the trenches there. He was about as far away from being a true “highlander” in the, supposedly, accepted form as anyone here (apart from one or two notable exceptions) but no-one questioned his entitlement to be wearing a kilt. Provided they are not required to wear a particular uniform anyone is perfectly entitled to dress in any way that they see fit and, as long as they are not causing offence in some way, no-one has any business to say otherwise.
I am sorry that Irish Jack O'Brian and others have felt themselves ostracised from kilt-wearing and hope that they will discover that there are many more native Scots (and others) who are only to glad to see the kilt being worn than the one or two curmudgeons they have encountered