Thing is, it's not that simple. What if there is only a ticket vending machine and you want to pay by RTV? What if the clerk says that the ticket you want doesn't exist when it actually does? How about if there's a permit to travel machine but your only coins are 1p and 2p (and you have enough money in notes)? What if you want a split? What if there's a gigantic queue? And so on...
If you've only got 1p/2p in coins then thats all you put in the permit to travel machine, there is no minimum requirement and if you've got no coins at all you don't bother. Certainly on the Medway Valley line the PTT machines are rarely used and buying a ticket on board or at the destination is the norm.
What gets me about this 'rule' about buying a ticket before boarding is that it puts the onus fully on the passenger and not the TOC to provide adequate facilities to buy a ticket.
On another issue I don't think it is up to the guard to be warning passengers about buying tickets first, it will just lead to arguments.