Hi, I think you must realise that the mills aren't made by the suppliers, just imported and supplied etc, and they mean to sell every one of them eventually......so that means all those that fall down on the quality issue will be re-supplied to the next buyer until in the end someone with the knowledge and expertise to rework one of these efforts does just that and makes it work properly, but in saying that it's assuming the final buyer also knows something about machinery and buys one anyway knowing that with hours of fun it can be made to work better.
I would also say that if the mills were reworked to a higher standard they would fetch a better price, probably $500 higher, but with a turnkey quality that guarantees straight out of the box working ability.
This is of course would be a total waste of time as the mills are not intended to be turnkey per se, but as organ donors for higher concepts CNC wise, and with that possibility the rework also becomes part of the rebuild if the builder has distinct mechanical knowledge beyond just bolting bits and pieces together like a Meccano set.
BTW, the mill in question only needs a few hours of scraper work to make it pristine, so with that knowledge gained from hands on experience, anything relating to machine rebuilding in the future is a feather in your cap.
Ian.