George told so in one of the earlier Contest threads, apart from that the details are always listed in the AUM contest section.

JCL is the boss, so he decides if the source code has to be made available or not for each contest. I like to look at other people's source code, but if it's a game that they plan to sell I can understand why they don't want to show the source. On the other hand, if they want to include the source code, they are free (and actually encouraged) to do so.