Mishpatim

Shemos 21,6-Rashi-We pierce the thief's ear(if he remains a slave after 6 years)because even though his ear heard on Har Sinai "Lo Signov"he stole anyway. But Rashi on Shemos 20,13 says that that the "Lo Signov" on Har Sinai is reffering to kidnapping, not theft. If so, why are we piercing his ear because he heard on Har Sinai not to kidnap? That's not what he did!