You're definitely not speaking about the UK as a whole, this is completely normal and rooted in not wanting to waste food.
I only see it happen if someone falls ill, is called away or there was an error in the order e.g. you manage to order four entire chickens instead of four portions.
Is it because a small paper box is involved? Would you find it less obscene if everyone carried a reusable food container with them to a restaurant to mitigate the risk of offensive boxed leftovers?
Many people eat more than they need to at any given time, that is arguably greedy and argubably wasteful in a different way.
Portion sizes are static, appetites vary, letting people manage for themselves is perfectly fine.