At first glance it looks a money grabber. But on closer inspection the Irish low coast airline has a point: to cause less disruption on boarding. If parents with kids under 12 need to sit together, which they should, it causes headaches for cabin crews to do just that.
Often those who paid and chose their seats are unreasonably being shifted around. If I had paid the 8 quid to reserve, I would be peeved if that happened to me. Why should I be inconvenienced? As if flying cattle class isn’t stressful enough already.