They just might not have fought right away. I think sometimes males will display for each other before they fight, and some are more aggressive than others anyway. Or they could have just been momentarily too freaked out. They were bred to fight, but that doesn't mean they have to start wrasslin' the minute they see each other.
I've heard of people keeping a male and female together for months before one decided to tear the other up. Not worth the chance imo.
I definitely wouldn't count on them not fighting in the long run.