It's not a problem if they deliver on time. The only reason so called "pre-mining" is a problem is when manufactures fraudulently advertise and early delivery date and then delay the delivery date in order to charge a higher price and still use the equipment to mine for their own account.
So true. While I would prefer to have an unused miner, the truth is these products aren't going to be running very long. It's very unlikely any pre-mining will effect reliability, and thus customer profits. I just want my unit shipped ASAP, which they do.