IIRC prosecutors are not allowed to selectively enforce a law. For example, if Asians are forbidden from being within Boston city limits then the government can't prosecute only Yao Ming to keep him from beating up on their basketball team.

Wikipedia:

[According to SCOTUS] "A selective prosecution claim is not a defense on the merits to the criminal charge itself, but an independent assertion that the prosecutor has brought the charge for reasons forbidden by the Constitution."[1] The defense is rarely successful; some authorities claim, for example, that there are no reported cases in at least the past century in which a court dismissed a criminal prosecution because the defendant had been targeted based on race.

Good find. The only time I've really ever heard this defense ever discussed in the mainstream media is with regards to the Linda Tripp case.