We attended a Princess & Pirates Party in April 2008. We were told we could arrive any time after 4.30pm but only enter through one particular gate. We actually arrived nearer 6pm as we'd spent the afternoon at Boggy Creek.
We were handed maps and told that after 7pm the treasure and other goodies would come out - but not before. And yes, it was very satisfying queuing up to go in one ride with some people without a wrist strap at 6.50pm and watching them come out at 7.05pm just ahead of us and then be turned away from The Laugher Floor as it was now gone 7pm.
It's not just a crowd thing: I was told that they sell 4,000 tickets to these parties. When I said that was a lot, she pointed out that no, they could get some 40,000 people through the gates over the duration of a day at MK, so you can see the attraction of a limited ticket event!
Over the duration of the evening there were lots of games, the handing out of pouches of gems and beads at different points, and some trading of different colour necklaces going on as well. We were never asked to show our bands, but as we were all wearing teeshirts they were very easy to see.
The main point is that any event that sells tickets and then GIVES AWAY things (such as the gems and necklaces or, in the case of the Hallowe'en party, candy) then why should someone who hasn't paid get anywhere near them? It's not just a case of buying merchandise they're not supposed to. The cost of the tickets covers the cost of the freebies.