I'd think they'd be happy you like their soup so well you don't want to waste it.
Sure makes for a nice meal for you if they are serving something you don't like.

They are abiding by the law only keeping it in a server for 2 hours.

I worked in a restaurant out of highschool and mostly did end of day closings.
There was always some leftover mac salad, tuna salad and blueberry muffins....owners were glad they didn't have to toss it, everything was made fresh every day.
My mom was so happy, she rarely had to go to the grocery store.