Literally illegal in many places.
Edit: including much of Europe.
Looked online and found maps suggesting eastern Europe has more laws relating to it, although many of them in practice don't apply
Which "eastern parts"? I've never seen that rule here, but have seen people drinking in public. Do you know that or are you just asking AI to confirm your biases?
Public drinking is generally illegal in Poland ('police take a strict approach'), Romania, Lithuania, Slovakia (apparently not enforced in Slovakia).