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.