Shouldn't the drive itself specify the capacity and the case is just an adapter?
Back before SATA, I had a couple IDE->USB enclosures that were limited to 128GB drives because they only implemented 28-bit LBA. I would be amazed, however, to find any SATA controller that didn't implement 48-bit LBA (for a maximum capacity of 128 petabytes). I can't think of any logical reason the adapter wouldn't work for drives over 500GB.
About 3-4 years ago, I was shopping for a SATA dock. Those that did list a maximum capacity were always 500GB or 1TB, which corresponded to the largest drives on the market at the time. I suspect they just printed the largest drive it was tested with, but some indication of future compatibility would have been nice. I forget what the one I bought was rated for, but I should have a larger drive to test in it before long.
EDIT: Just looked it up, mine says 1TB max but I have used a 1.5TB drive in it. So I'm going to stick with the presumption that they're only claiming what they've actually tested.
Since there is a SATA-to-USB-Mass-Storage controller in the box, that might be the limiting point - the "adapter" is in fact an active component, not just a jumble of wires.
As you can see, the box is cheap, so my assumption would be one of these:
a) the controller's firmware might be ooooold and not work well with larger disks, or
b) the manufacturer hasn't bothered to test with anything >500GB (time is money; good for you if it works with larger drive, don't expect support if it doesn't), or even
c) these "limited" controllers have failed QA for larger disks. This happens e.g. with CPUs - a unit which fails QA but runs OK in a limited setting could be sold as a lower-class model.
Yes you are correct that the drive within the case specifies the size. I don't understand your question though... 2.5 USB cases are limited in capacity:
because 1TB 2.5 in drives only recently started hitting the market. In fact Western Digital came out with one about 3 months ago. So if that external was manufactured 6 months-1 year ago, there wasn't even a 1 TB 2.5 inch drive to put in it.
Power consumption: these drives are meant to be portable. So they usually don't run on AC power and take into account that laptops/netbooks don't have an over abundance of USB connectors. For this reason they are usually limited to one USB plug for power and data (some have a Y splitter 1 for power 1 for data). Larger drives require more power than a single usb port can some times provide.
In case you are just confused about the term "max capacity". What they are saying is that it can take a 500GB drive but since no drive is created with exactly 500GB available and usable it maybe 489 or 492 GB but at the most it will be is 500GB.
The Conrtoller: As Piskvor said it may just be that the controller is limited by firmware to 500GB. This may be because it was manufactured prior to drives larger than 500gb being produced or that they want you to buy their higher model enclosure for larger capacity drives. But this is just speculation and since amazon states: "Brand Name: Unknown" there is no way to confirm this.