Because that is the way the government is allowing them to operate. Called rules and regulations, just as with the auro industry, the meat packing industry, the firearms industry, etc., etc.
BTW, buying a hog at auction is a bit of a streatch of calling buying your groceries at an auction, but you may feel that way if you wish.
Buying groceries at auction is buying loaves of bread, canned vegies, product, etc., etc. Thats buying groceries.
Or do you go to the public auction house once to bid on items similar to the above?
BTW, be watching for the increase in gas pricing because of federal regulations.