I also get you OP
I have a friend who constantly lets her DS's have coca cola and huge bags of sweets when we are out, they are both under 6. I automatically will say no to my child as there is no way, in my view a child of under 6 should be consuming that much crap, at soft play, in the park, in a café, whatever.
Then they are hyper and whine and whine.
I can't help my default telling off, it just comes out. My child is only allowed wáter, it makes me look mean and prissy, which I am not. I always come home with a massive headache and worrying I have been a bitch.
I hate it, but love my friend. My child always goes quiet when hers start screaming and demanding, it is upsetting for everyone.
OldBag sums it up perfectly...
It's awful having to explain to your child why you won't let them do the thing the other DC is doing - especially in front of the other parent.
The other day we went for lunch, we had icecream for pudding. After lunch we went to the park, one child asked for icecream, I couldn't help myself, I said' no, you just had icecream', it is what I would say to my own child if asked. My friend bought him icecream 'as he was probably hungry as he ate nothing at lunchtime except the icecream' 
Child ate half then threw the rest away. It is like this eery time we meet.