Pinchas

In Sanhedrin 82a, How could Moshe tell Pinchas to kill Zimri ("kreina di'igrisa ihi leihavei parvanka" [the reader of the letter should be the one who fulfills it]) if (Sanhedrin 82a) the Halacha of kanaim pogim bo (zealots kill one who has relations with a gentile); is Halacha V'ain Morin Kain (its the halacha however we don't encourage people to do it)?