I think Rockstar intentionally made it that way to make Portland missions more harder, and to discourage you from going to Portland. Also, the mafia are armed with normal shotguns, not Spaz 12s. Spas 12s were introduced in GTA Vice City. And the Mafia are only hostile to you after the completion of "Sayonara Salvatore", not "Last Requests". In order to achieve 100% completion of GTA III, you complete the Portland missions before that mission, as it makes things easier.
Sorry, i mixed up the 2 final missions and the shotguns do look like spazes.but still,i loved portland and if someone wants go around portland, he better steer away from saint's mark (which was the best place to have fun with the police)and i still want to go around portland without being turned into a million pieces when i haven't done anything