If you boarded before the tickets for reservations were put out, you might find yourself having to move - and by then, you've got other people who were 'lucky' to go into the unreserved ones.
Therefore, I can see why a TOC would want to keep people off and surely they have the right to keep you off, even if the doors are open?
By comparison, FCC will let you board a train and sit on there for over an hour at night when they're down to the last few services! There might still be someone coming through cleaning, but they won't say anything to you - except perhaps giving a smile of appreciation to those who pass items to him/her.
A lot of them speak very poor English, because most English people feel such a job is beneath them, and some may say things that sound abrupt but isn't intentional.