BEagle, the practice of the crew boarding their aircraft from same boarding gate as the pax has been such at least since 1988 when I started travelling to and fro.
I also agree with your view of good manners. But, sadly, you and I know that they are in short supply. Manners are, of course, a 2 way street. The pax in this instance paid over £3000 for a return ticket. I'll bet a pound to a penny the Captain didn't know. Did he care? Probably not. But it is not enough to say that the Captain thought the guy might be a problem once on board. Maybe, maybe not. But I think he probably overstepped the mark.
Did VS no good at all though.