Ergonomics: If the roll is beside the seat, it is typically installed at or below the height of my leg/knee. Therefore, in order to get the paper if it is hung “wrong”, I have to go over my leg, under the roll, then pull the paper back up over my leg. Compare this with the ease of pulling straight off the top/front and it’s clear which design is more user-centric.

Efficiency: For each sheet unrolled, you need to move much further in the “wrong” scenario–in the “right” scenario you can actually stop unrolling with the seam on top of the roll, for minimum work, in the “wrong” scenario you need to unroll at least enough extra paper to leave the seam hanging down under the roll. The amount of extra unrolling is proportional to the current circumference of the roll. (If you look at the two pictures above you can see this simply: I can get a sheet off the “right” roll without doing any unwinding, but to get a sheet off the “wrong” roll I need to unwind at least another sheet worth of length–and probably more due to the ergonomics issue.)