If one of my boys did something like that, I would be saying something to the person who bought it for them, something along the lines of "I am SO very sorry they misled you!" While taking the beers from them and telling the bartenders they are not to be served any more alcohol.
And the boys would be getting the "You are SO getting an earful when we get home!!!!!!"
It sounds like it was easier for the mother to be nasty to you, instead of dealing with her own daughter who had (A) lied about her age and (B) accepted a beer under false pretenses, knowing she wasn't of legal age. Since the daughter did all this in full view of her own parent, it seems apparent that she's used to calling the shots with her mother.
Your error was completely innocent, in good faith, and very understandable -- you did nothing wrong.
I was thinking that, too.
I suspect this is the cause of the dressing down, personally, not jealousy of the OP or anything else. I have seen parents who have no control over their children take the "well then I'll just have to control the whole rest of the world" route. She can't control her daughter, therefore she can't take the drink OFF her daughter, therefore it's the OP's fault that daughter is drinking
I'm with Amava - I got up to plenty when I was young, but never when my mother was right there. No way, no how.
Short answer: You were fine, the two women were wrong.