Good. The rule is stupid and deserves to be worked around. Water is essential. I hate when I go to these kinds of public places and they give you no option to get water other than bringing it yourself. Poor logistics on the part of the event organizers.
It doesn't look like the rule is from the event organizers. Most likely local government who want's the money from the event while keeping the turnover high so that people will come, spend their money, and fuck off as soon as possible.