I don't know for sure, but I guess back in the day, in particular in Brazil, they didn't really make a difference between pro and ammy fights. You fight, you get money for it, that's it. At least that's what I remember when our guys went on free fight tournaments at that time. "MMA" wasn't even a fixed term then here, nor was it really a pro sport.
Its a state by state thing although I think NSAC is one of the few that are not cooperating with the unified rules. Ive been to the amature mma events here and I thought they were all five minute rounds but like you im unsure.