Was this for randomly selected participants? I want to participate and get $100. That sounds like a lot of fun.
Did you get invited? Or know how to get invited to participate in these events?
Some exchanges are also offering that kind of competition. I saw some that gives you trial funds that when you could reach a quota for trading volume or gathered/deposited amount then you now could withdraw that trial fund. It might work this way and the given 100$ for trading could only be withdrawn if only you could win the competition plus the prize. Nothing free either.