Not least because 10 - 4 is now the norm for quiet weekdays,
Opening times are still very dire for such a popular theme park. Not only have we lost time at the end of the day, but we have also lost ERT time too. At Alton, you will spend a good hour or more queuing (in total not per ride), even on a quiet day, you will spend a good hour or more walking from ride to ride, time you take to eat, even on a quiet day, the opening times are not great for such a large place.
If someone does turn up and pay gate price knowing the park closes in three hours time, that's their own decision, after all.
You just can't assume that people turn up late on purpose or due to bad planning/organisation. There are many reason why people don't turn up to a park for opening time. I've been nearly caught out in the past by traffic, break downs, road closures/diversions (especially on the A14 for the past few years), family or friend that is coming with you that has bad time keeping etc....
The further you have to travel, there is less room for error and the more chance that your travel time can unexpectedly increase.
I'm not justifying people for not first checking the web site or arriving late, but the fact is that many guest do not check the web page and many will arrive after noon. My original post was a light heart'd comment about people paying on the gate prices for a measly few hours, I didn't expect this to turn into a debate about how people should check the internet first. Might sound a simple thing to do, but not everyone will. And if people do take a look at the web page, the closing times change from the weekend to midweek, people may take a look at both times but for some reason only remember the weekend time and then get it in their head that the closing times are open longer than they really are for mid week. Before now we have all got confused over information, I certainly have.
I don't always check the web page for closing times on any of my visit to any of the attractions (maybe my local zoo or a Merlin attraction further afield). But I always operate a policy if I can get to a park early and for opening time, if I had enough of my visit, I can always leave early, but if I arrive late and I hadn't had enough time, then I can't gain those loss time at the beginning of the day for turning up late. But not everyone thinks like you or me.