Well, in that case I don't think that these are copies at all. They were probably manufactured as genuine SIDs back in the 80s, but discarded at some point due to quality control. Somehow they were not destroyed at this point but ended up in the market where they were sold after being visually refurbished to look like new. This is how it is with the other 'fake' SIDs out there.
Setting up a new production is a very costly operation and I don't think they can sell enough to make it worthwhile (they would probably need to sell >100000). They might also have to do a redesign since the silicon process used in the original SID is no longer available, and processes that can do mixed A/D are difficult to work with. If it had been a purely digital chip, it would have been easy to copy.