It wasn't rigged. People don't understand boxing and entertainment. It wasn't a "real I want to punch this guys head off and kill him" match. It was a boxing match. An entertaining match with 2 people beating each other.
Tyson (old) can't go the distance anymore. He came out hard but got tired QUICK. Paul was able to eat the abuse and hit back. Paul could have knocked him out, but slowed down and danced around Tyson when Tyson got tired in the later rounds.
They both did it for entertainment and to make money. I'm not sure why anyone is surprised. It went the way many people predicted.