Oh right, since it came up in PMs. 

If you're having trouble coming up with motivation for your character, the rewards list is here:
Loot, honor, actual fame, possibly the rewards of becoming a celebrity when you get back home, obviously cash because the higher your kill count the higher your payoff.

So there's a good list of reasons why they would want to enter.

Edit: I've reviewed the character sheets that have been posted so far.