I think there's actual logic here. If you're storing a liquid, you're probably going to be getting it out with a big spoon, so the round sides make that easier than flat sides and corners. If you're storing something like lasagna, you're probably cutting servings into squares/rectangles, so the flat sides make more sense.
As someone who cooks large batches of food to freeze, the problem with round is that it wastes tons of space in the freezer. I just use scores of rectangular cubes and my freezer is completely full.
That's always the trade-off. Square ones fit better. Round ones are easier to wash (the corners non-round ones are slightly annoying to deal with depending on what was on the tupper)
If you're storing a lot of liquid you'll probably want to pour it out later. In that case square is better as the corners will provide a bit of spout action.
This ladle is reinforced silicone in the back half of the scoop, and soft, flexible silicone on the front half, so you can scoop into corners easily. It's very nice, even for cylindrical containers because there is still a corner where the wall meets the bottom.