It could be that they were both reasonable people and realized that a dating relationship wasn't right for them, but they enjoyed being friends. Unless there are warning signs pointing in other directions, this is probably the best approach to take to avoid creating a problem where there may not be one.
It could be either of those, or that he opposed the idea of cutting them out of his life completely. Another possibility is that things ended in a civil enough manner for his past relationships where a friendship or acquaintance-level connection with the old girlfriends could still be maintained. Personally though, that's not my style - my exes are just so for a reason, and they're out of my life totally.
And what's about he's unable to stay friend with them? Does it mean that he still hurts him to see them..and that he still has got feelings for them?
What I mean is I think it can reveal that he likes human contacts and he's able to not stay in a bad mood about what happened with his exes. As they are exes, it's done with them but they can stay friends.