As you say there is regularly 20 plus people queuing at the excess ticket desks. Its seems to have become common practice at Cardiff Queen Street to pay upon arrival at the station due to the limited ticket facilities available on the valley lines. Especially, at the weekends when more than often enough the ticket booth is often locked, which is not helpful especially if the conductor on the train is hiding in his cabin or is unable to do his rounds on the train as it is too crowded.
For example on Saturday morning, when I went Christmas shopping there must of been 40 people queuing up the corridor towards the desks as I walked past. Surely Arriva must realise they have a ticketing problem at the valley stations with people being unable to buy tickets? Also why only pick people at random to prosecute, if one, why not all? or at least put up a warning sign to make people realise they must buy before travelling from this point forward? Judging by the queues Arriva appear to be unknowing providing a ticketing service upon arrival at the station.