Why shouldn't I, or anyone else, sit there? There's no rule to say I can't. And as you say they offer more leg room which is at a premium on some trains, and I'm not a really tall person
How do you know someone is in that category? It's not always obvious, and getting it wrong can cause the person to get aggravated, like I said earlier, and also I don't know about you but I'm not constantly monitoring who's getting and off the train at every station.
And why can't they ask someone in the priority seat to move?
I agree there is nothing to stop them seating there, however if they are sitting there they should offer the seat to someone who qualifies as a user of that seat. That does of course mean on most journeys they won't have to move, but occasionally they will.
Now of course they may be in a priority class themselves, however they are not old, not pregnant (usually male!) and from the way they push and shove to get to the seats clearly not disabled. Not forgetting I am seeing these people travelling on the train everyday.
Obviously those who need the priority seats could ask them to move but invariably they dont, either someone in a non-priority seat offers to move before anyone is asked or those entitled to them asks someone in a non-priority seat to give up their seat.
Regularly catch a Southern train in commuting hours and you'll see this happening. All it really needs as someone mentioned is for those entitled to seat in the seats to have a badge or pass and to be told to ask those in priority seats to move first. If they find they are entitled to them they can then ask those in non-priority seats to move.