Different cultures and religions have different rules. If a person wants to eat certain foods that doesn't mean they can then impose that on others. It's up to that person to stick to their own rules not to judge the rules that others live by. Therefore there is an inadvertent judgement in the question here. "I don't eat fish therefore you shouldn't " etc.
Personally I don't eat fish because it smells like a toilet but I don't try to stop others eating fish (interestingly fish eaters agree that fish smell like a toilet but they still eat it!).
Different vegetarians also have various rules. Some eat seafood. Vegans don't even drink milk; and there are endless variations each with differing arguments as to why theirs is the best way and why others should follow their way.
Indigenous people often have a respectful or religious association with their foods especially animals. In certain cultures certain tribes can't eat certain animals but can eat others.
The answer to your question is therefore not to be too concerned with what others are doing and only be concerned with your own practices.
In the time of Jesus in that poor environment people ate fish to survive.
It was part of that culture at that time.
If we concern ourselves with what others are doing we might miss the chance to practice tolerance and understanding. If we don't concern ourselves with others this "question" just disappears or becomes irrelevant.