This isn't Target's fault. It's an intentional feature of the chip card.
With a magnetic strip, you could swipe the card and the terminal would store the information until the transaction was ready to be submitted. Incidentally, this is how Target was breached: the info on your magstripe was stored in memory, unencrypted. Malware installed on the point-of-sale terminal was able to snag the info and send it outside Target's network.
In contrast, the chip in your card is actually used DURING the authorization, to avoid a man-in-the-middle attack. The authorization request can't be made until your transaction is complete (and the amount is known).
Yes, you could insert your card into the slot before your transaction is complete. But, some merchants are not allowing it, so they don't have to worry about someone forgetting their card when checkout is complete.