There is nothing wrong with sides on the base, even tho it's less strong it is a neglectable weakness. But in theory side on top of the base, the water pressure pushes the panels out, which creates a rolling edge force on the bottom seams. And silicone is weakest in holding this rolling-off force. Silicon is much stronger withstanding a pushing force.
And as explained above it actually is more expensive, because there are 4 sides extra to be polished with bottom outside.
ANd it's more difficult to assemble because you would need standoffs for the side panels to maintain the prefered seam thickness. Because the weight from the panel will push vertically on the yet not cured silicone and squash it out. Thus if you have a well build bottom outside tank it actually prooves good craftmanship.
That should and could be respected.