Someone hit the nail on the head, it is learned, either from your parents, relatives etc or you are self taught. There are a myriad of resources available for those that want to learn. There are those that put their own interpretation on what black/white tie means and their reasoning usually comes down that they want to be an individual.

From my own experience I would say that the general formality of dressing, whether it be for casual of formal events declined in the late 60's early 70's for a majority of the western world. One only has to look at old TV programes of the 50's & early 60's to see the different level of dress as against those of the later decades.

I am not suggesting that the change is wrong, but there is a way, even in this day and age, to dress in a correct manner in relation to the level of formality of the function that you will be attending.

If you don't know how to do it, then it's up to you to learn.

There are always some who want to stretch the boundaries, sometimes it's accepted, sometimes not