The thing is people say "in moderation" or "on special occasions" but those are pretty vague concepts i.e. one parents special occasion might be Christmas or New Years, another might mean family parties, exam results or anytime the teen has friends staying for the night. And moderation for one person might mean a small shandy, for another it's 3 cans of cider.
Honestly I think many parents are well intentioned (and some are surprisingly blasé) but it's one thing to say they are allowed a Buck's Fizz on Xmas day and a glass of champagne on NYE, there's no ambiguity there. When it's all a bit of a moveable feast as it were, and there's a vague ok to drink sometimes you've left the door open for your 14/15 year old to argue that this is one of those times.
We were invited to a barbecue during the summer, 50th birthday, it was a family style party i.e young children, teens, adults. A few of the teens in attendance, I think two were 15, one 14, were kicking off with their parents arguing that "well you let me drink at x thing or y thing, why not at this" and "how come it's only when you decide something is a special occasion? I think uncle X 50th is a special occasion". It probably didn't help that there were two others the same age who were allowed (and a couple more who it wouldn't even have occurred to). It was actually a bit awkward as my friend who organised it hadn't expected that there would be teens drinking or looking to drink in her home and a few people felt uncomfortable.
I just think that when you start introducing it to young teens you blur the boundaries so it becomes very normal that they drink and as pps have said this doesn't mean they'll be fine at 16/17 when they're being offered and encouraged to drink all sorts by their peers. UK and Ireland (where I am) have major issues with alcohol and I'm not convinced giving it to children or teens is the way to address the problem.