There's no rhyme or reason as to how they mount those outlets: prong on bottom, prong on top, I've got at least two pieces that have them sideways.
If you're only going to use two prongs quit being so cheap and stick on a two prong connector or just hard wire it in there and then you can complain about no detachable power cord.
Frank Van Alstine was funny, he said he finally put in a detachable power cord socket so you can buy your own fancy one and it can fall right out!