It's not considered to be a sin. The Jewish do it because the old testament commands it so.
9 And God said unto Abraham, Thou shalt keep my covenant therefore, thou, and thy seed after thee in their generations.
10 This is my covenant, which ye shall keep, between me and you and thy seed after thee; Every man child among you shall be circumcised.
11 And ye shall circumcise the flesh of your foreskin; and it shall be a token of the covenant betwixt me and you.
12 And he that is eight days old shall be circumcised among you, every man child in your generations, he that is born in the house, or bought with money of any stranger, which is not of thy seed.
13 He that is born in thy house, and he that is bought with thy money, must needs be circumcised: and my covenant shall be in your flesh for an everlasting covenant.
14 And the uncircumcised man child whose flesh of his foreskin is not circumcised, that soul shall be cut off from his people; he hath broken my covenant.
But you know, Christians are all hip and cool and don't bow to the old rules. Jesus fixed all of that when he invented the new testament.
... but for some reason, Americans still like doing it.