It's been to my understanding (my friend is a promoter for Blueprint Events) that water bottle caps haven't been given out in the past for the following reasons:
1) Security at these things are very worried about "weapons". I don't agree with it, but that's why they don't let you bring in those thick glow sticks. A full water bottle with a cap on it could be thrown around and hit someone and hurt them. But if it doesn't have a cap, the water spews out and it becomes less dangerous
2) A water bottle, regardless of if it has water in it or not, with a cap on is very dangerous if someone steps on it. Considering the thousands of people who are dancing and how dark it is, it becomes easy for anyone to roll their ankle or trip
Again, I can't say i 100% agree with this, but for the promoters and the venue it's safety first. The first time I noticed them starting to give out the caps was at Life In Colour (beginning of September). IMO, from all the deaths that happened during festival season in the summer, they decided it was best to keep people hydrated better