Because you paid using a credit card/debit card/store loyalty card.
So you can pay with cash. But soon cash itself may have these chips too.
...the mu-chip is a Pandora's box, believes Lee Tien, an attorney at the Electronic Frontier Foundation (EEF), a San Francisco-based organisation that specialises in the conflict between technology and personal rights. Although he has no intention of demonising the Hitachi chip, it is nevertheless an example of how surveillance technology is getting cheaper all the time.
Overtones of Big Brother may not be the only problems. If tiny chips woven into money and other valuables were constantly announcing their whereabouts, a thief would know precisely which person or home to rob. By the same token, chips in clothing—linked to their owner's identity at the time of purchase—could mark the wearer's location anywhere on earth.