There are a number of festivals, like the one I was at last weekend, who do not allow you to take your own drink onto the site and therefore you have to buy what that particular festival offers. However having been in the soft drinks trade for over twenty five years (although we produce more flavoured spring water than plain)I have to say that all our plastic (PET) is recycleable and if possible should be disposed of in the appropriate containers.
On the other hand what about all the festivals who now serve drink in plastic glasses - even when boasting a real ale bar!