I agree with
@BeTeP. They are probably be using a Zener diode since using only resistors wouldn't work at all. Zener diodes generally make for terrible voltage regulators when the load varies; they can work for powering small static loads or as a cheap voltage reference, but in an adapter like this I can't imagine it working too well. Making matters worse, again as
@BeTeP alluded to, the likely failure mode of Zener diodes is to fail shorted. Even though OKGear appears to stepping down the 5v rail in this adapter, in the event of a diode failure, the series resistor alone won't keep the voltage below 3.3v and it will likely fry the drive.
Random OKGear story (unrelated to these particular adapters):
Years ago I was building my first NAS in a Norco 4224. I bought OKGear Molex splitters to provide power to the six Norco backplanes. Fired the server up and noticed a "hot" smell. The drive I have in one of the hotswap bays wasn't showing. Tried a few more drives; nothing.
The OKGear Molex splitters were wired wrong and swapped the 12v and 5v rails. Every backplane was fried. The few
brand new Seagate Barracuda XT 2TB drives I plugged in prior to realizing the issue were fried as well. Hundreds of dollars in damage caused by cheap, simple Molex splitters.
After OKGear never replied to my emails (I wasn't rude or anything; I mainly wanted to report the issue and see if they'd cover any damages), I vowed to never purchase an OKGear product again.