They do that because everyone recognizes that there is a very healthy market for classic characters and toys...except Hasbro.
Good thing that the box has differences that can be easy to spot. However from my own observation of people who buy G1 KOs, they don't seem to care if it's legit or not. At the Parra fair, they even get annoyed if you mention it or give you a 'don't care' expression. On the Internet, I have seen people saying stuff like 'It's just as good'.
Yeah there are those who get caught and buys the KO unawares but I find the type of people who don't care or deliberately buy KOs to be more common at least from what I can observe.