I totally agree with you OP about other parents not giving your dc something that you may not give them at home - same would apply to an activity that you might not let them do, or being in a situation (e.g. let overnight at home) that you wouldn't agree with. It seems like it should be courtesy and common sense to ensure all other parents know about it, before it happens.
I also share concern that teenagers are 'taught' to enjoy alcohol by trying alcopops - you can't imagine how much of the alcohol industry budget was spent on developing and advertising sweet drinks which would appeal to young drinkers and would disguise the taste of alcohol. They don't do that without a good commercial reason - one which builds brand loyalty and keeps people buying alcohol. But disguising the taste does not disguise the effect that alcohol has on the brain, the heady feeling of intoxication, the feeling of being a little more confident, and the hope that another drink will add to that effect (which ultimately it doesn't). In effect we are also teaching our dc that every celebration, is only a celebration if it is punctuated with alcohol, and that in order to let your hair down and party, you need to have a drink.
I remember from parties in my own distant youth, and from my dc's more recent 'gatherings' that events where some alcohol was provided or permitted was viewed as a green light, and in addition to the few cans of beer or bottles of WKD there would be bottles of vodka. I know how good we were at being deceitful so that the parent(s) in charge didn't know about the vodka and how they would pride themselves on our 'sensible drinking' - when it was anything but.
I am not impartial or dispassionate, I have suffered for many years with an alcohol problem (now under control for the last 4 years or so). I wish my own parents had not introduced me to drink when I was young - I'm not blaming them, my drink problem is my own responsibility and I may have gone off the rails with my first legal drink at 18 anyway. In understanding my own problem I cannot see alcohol as anything except a toxic substance, one which had mood altering effects and which has the potential to damage our body systems as it processes it. I wish I had not given my own dc alcohol. They do not seem to have an alcohol problem but I can't understand why I would have willingly introduced them to a substance like that 