How could they offer Korbanos? After Yehoshua died, the Mishkan was established in Shilo, and Bamos were forbidden!
R. Shimon (Zevachim 119a): "Ki Lo Vasem? El ha'Menuchah v'El ha'Nachalah" - both of these refer to Yerushalayim. There was no Isur Bamos when the Mishkan was in Shilo. (This is unlike the Stam (majority) opinion. - PF)
Meshech Chachmah (Devarim 12:8): This was right after Yehoshua died. Bamos are forbidden when the Ohel Mo'ed is erected, and the Aron is there (Tosefta Sof Zevachim). At the end of Yehoshua's life, he gathered everyone to Shechem "Lifnei ha'Elokim" (Yehoshua 24:1), i.e. they brought the Aron there (Rashi 24:26). Since it was not in the Mishkan, Bamos were permitted. 1
A Hora'as Sha'ah permitted Gid'on (6:19) and Mano'ach (13:16) to offer (Temurah 28b, Zevachim 119b). Perhaps also here was a Hora'as Sha'ah. (PF)
Meshech Chachmah: Similarly, after Pilegesh b'Giv'ah, they offered Korbanos in Beis Kel, for the Aron was there (20:26, 27).
