If you look at the case with that side being the bottom, as cases were originally designed, it makes more sense that it isn't considered removable.

Even my $350 case (that price includes PS and SAS backplane), the "bottom" can't be removed. Though the drives all get mounted into a carrier of some sort than that carrier locks into the case. So I can use four screws on each drive.

Pioneer PDP-5020FD, Marantz SR6011
Axiom M5HP, VP160HP, QS8
Sony PS4, surround backs