Once you have ordered a meal and it has been served, it is yours.
The restaurant does not see it as waste if you do not eat it, that waste is yours.
Waste to a restaurant is food that they purchased but did not manage to sell.
Your meal has been sold
So what happens to the food that you ordered but do not eat, is entirely up to you.
If you go to the store and buy, say, three shirts, pay for all three of them and leave one in the store on departure and never come back for it - most people would call this nuts, but they see no problem with doing the same when ordering food.
This is really odd, considering.
If you hate to see your food wasted, bring a small plastic container and take it with you. Or ask for smaller portions, or only half a menu. You have 100% influence on the food that you choose to waste, or not.