If you are going to read terms and conditions, its important to understand that copyright policies and usage right policies are two different things and should not be confused as being the same.
You can enter the vast majority of photo contests out there and still retain your copyright, but you do have to read the small print as some competitions do have unfair usage rights, where they can, if they so wish, use your photo for what ever they like for as long as they like, but each contest will have there own determining factors as to whether you should enter or not, even with what could be seen as unfair usage rights.
For example, if you win a $25,000 prize, I would think it very fair to let the contest organiser use the image, after all, if you was to sell your photo exclusively to them, how much would you get?, this is a choice for each individual photographer to determine the value of their work as it has been since the inception of the photographic industry.
But remember when entering a contest, you are entering in to a contract, so read the small print and pay particular attention to the copyright and usage right policies.