I can't obviously comment on the atmosphere during or after the game or the actions of a supposed few Leatherhead fans (a bit worse for wear from drink ?) as I was away and can only observe the posters comments.
Games for us in the northern league at P.P. shouldn't appear to be a problem as few away fans will travel and after the initial culture shock of having to pay £9 admittance and £2 for a programme (The prices at Wivenhoe and Ware £6 not sure about programmes) and no opportunity to buy a burger

, most of the dozen or so fans at each game I'm sure would be grateful of a chance to have a drink in the bar before the game and would probably like a drink afterwards provided they can afford it.

If around 40 Leatherhead fans travelled to the game I would have thought that with both bars apparently open and only some of these fans requiring a drink we could have more than accomodated them, but perhaps by design the security deliberately didn't point out the function room bar to eleveate the possibility of trouble.(ref. TLD's comment)
As we progress further in the cup (Sittingbourne for starters) we will definitely get alot more travelling fans to games at P.P. if drawn at home and surely we can be a bit more hospitable towards them until numbers dictate that segregation is required.(i.e. AFC Wimbledon & Dover etc).
The northern teams just like the southern teams welcome the Darts 100-150+ fans to their grounds, ( xdover maybe the exception ?) primarily for the amount of money we spend in the bars as well as providing a good atmosphere and as far as I know very little trouble if any has been caused. (And we often have a few that have indulged in one too many drinks)
I am not judging the security in any way, only the impression this action creates to Leatherheads fans and the message it conveys to lots of other travelling fans who logon to this website.
Take their money at the gate, sell them food and programmes, but also welcome them in the bar where their money is as good as anyones and it's no more than their fans deserve, not just before the game but afterwards as well, it's exactly what we would expect at their grounds
.(subject of course to the correct standards of behaviour)