Law is anything but absolute and this isn't even a law we're discussing but a policy or administrative rule.
There is the concept of case law which changes somewhat with time and is where judges and juries decide what the law really means.
What this means to me is that sometimes laws and policies may not apply as the letter of the law but if I choose to ignore one I do so at my own peril. Those in charge may decide to disagree with my version and I'll accept the consequences.
Hence, while you could get in trouble for a single beer in your campsite you likely won't and that is kind of the point of the rule.
It's also a tool available to those in charge to take action without claims of discrimination. Rules are rules they can claim...until someone says they enforce then unequally. ;)