When I heard about this last year, I thought it was awful, but on reflection, I can see why they do it.
Firstly, it is clear that this is the approach. No-one is left in the dark unless they don't read the paperwork.
Why do they do it? It's because they are on a tight space city site and need to fill to capacity but cannot overfill. Due to location, very large numbers apply and for many, it is not a serious option but very much a back up in case people don't get a state grammar offer, or an alternative independent. When the school makes offers to these vast numbers, they have no way of knowing who will take up the offer and who won't - there is more uncertainty for them than for many schools. So what can they do? They can offer to the exact number and then use a wait list after state school options day. The problem is that WL don't deliver very good solutions for schools if they have to recruit a sizeable number from them. Firstly, after national offers day, large numbers on the WL have decided on either another independent and paid their deposit or have selected a state school. The school finds they have to go down and down the wait list to people they would prefer not to take, in all honesty. Some of the people who say 'no' on the WL at this point, would have said 'yes' if getting an offer in the over-offering of exploding offers, because it was their choice, but later they are swayed by an alternative offer.
By using the exploding offers, the school can over offer without fear of having too many accept which they simply cannot accommodate. They know that those who really really want the place and for whom it is absolutely their first choice, will say 'yes'. If they can fill with them, they are happy. Anyone who was never serious about it, simply won't accept and pay. And those who really want the state school place and think they have a good chance won't accept either - and the school are fine with that, because the vast majority of them were never going to come anyway. The fact the school will lose a few of those who they wanted and then didn't get the state school places they hoped for, is a cost the school is willing to bear, becaue of the benefits overall they gain. And they judge it is their duty to the school and the current and future pupils to deliver that, more than to allow people to hold multiple offers or spend ages deciding.
So now City is full. Other schools have no idea whether they have over offered or will fill up....they won't know that until after national offers day. Many of them will then find they have to put on expensive bulge classes if more accept than they hoped for (City has avoided the need to do that) and others will have to go to their WL, or even to people who weren't even interviewed in order to fill up, taking those of lesser ability (CIty won't have to do that)
I agree that for people waiting for State school offers its very hard.....but the info was always out there. It's no surprise. For those with other independent offers, to be honest, I have no sympathy if they missed out - City is challenging the accepted culture of people sitting on multiple offers for weeks and delaying making decisions, which actually then affect other families too, because wait lists cannot move. It isn't unreasonable to expect people to have basically made a decision before the results come in. It isn't unreasonable to expect them to commit to it quickly. They can then release their other offers and those schools will perhaps offer places to WL people - they will be indirectly benefitted by Citys speedy policy.
In the end, having a lomg acceptance period creates uncertainty for schools and often for families who are hoping for a WL place and don't have any offers. Having a long acceptance and sitting on multiple offers was the culture, but perhaps it's going to become something of the past, because it creates huge problems. There is no obligation to indulge this previous practice of people having ages to decide. Parents feel outraged, but Citybis full of people who they were prepared to offer to. It is done and dusted. The parents who have accepted know that their cohort will be full of other families who had absolutely chosen City and who the school had chosen too.