While I think I understand that their point would be to encourage new members, in actuality, it may have the opposite effect, ie, people who have put their time into the community and have shared their triumphs and losses feeling passed over and frankly unappreciated.
The idea is to encourage new people to join the community and become active members. Opening everything up to everyone really offers no incentive to want to stick around to be a part of the community. Many forums have areas accessable to members over a certain post count, etc. The reasoning is that it gives people something to strive for and to want to become a part of.
I seriously do not think new people would not want to join the site because they couldn't join a contest. There is a desire within humans to belong, we have long been fascinated with the idea of belonging to a group from a young age. It often starts with girl scouts, boy scouts, 4-H, etc, then later into fraternities and sororities, finally, country clubs, golf courses, social networking groups, etc.
From an advertising standpoint, it would make more sense to encourage the lurkers to boost their post count and become a more active member.
The last round of photography contests were jaded in my opinion with the same issues which is why I participated once and really steered clear.
I think that limiting these forum contests to forum members who are over a certain post count is also a way for the owner of the site to thank those who do frequent the site, offer advice, and officially breath the life into the forum.
Just my $.02.