Let's be clear here, these folks are remanufacturing your iPhone, they are not supplying replacement cases. Technically it's not legal to make and/or sell the logo'ed shell. Apple protects its logo'ed products quite fiercely.
Frankly, some of these mods while some are quite handsome weaken the structure of the phone. So you'll end up getting a case to protect it and offer some additional support so your expense of tricking out your phone will be losted as only you'll know what you have hidden with the case. Lastly, if your phone had issues Apple would likely not fix it.
I would look for a tricked out case and spend my money on it. Unless I had very big bucks to throw for a different phone each week.
Apple is the only source for the part legally (remember it has the Copyright & Trademark Apple logo on it. Apple has had the police raid quite a few American & foreign stores & shops collecting the illegal products and having then destroyed. So I find it hard to believe they are still in business!
What these folks could do is buy used systems strip them down and then remanufacture them. As long as they can prove they bought real parts then modified them they can legally resell them. If that were the case the cost would be much higher than what they have listed.
Again, they can't make (mill) the case them selves or buy from other countries the case. Besides they may not find the exact match of raw material nor have the means to harden it (copper & aluminum both are hardened by pressure unlike steel).