Read the conditions carefully. As Stocky said, a lot are "rights grabs" as an example
Clause 22 of the conditions of entry
for a competition by Relationships Australia
reads:
Each entrant assigns to the Promoter all copyright and other intellectual property rights in each photo and description sent in as part of an entry in this competition. Each entrant acknowledges that the Promoter, as owner of the copyright in the photographs and descriptions, may reproduce the photographs and the descriptions in any media or marketing for any purpose without permission or payment. The entrants agree to waive all their rights, including moral rights, in the photographs entered by the entrants in this competition.
In this case the rights grab is for the promoter, who isn't Relationships Australia, its News limited, one of the newspapers associated with Rupert Murdoch.