If I had been planning to submit, this would be an automatic "no" for me.
"If a submission is selected as a prize-winning work, each entrant acknowledges and agrees that the submission may be posted on the Service; and entrant(s) grant to Sponsor a non-exclusive, worldwide, and royalty-free right to use, copy, reproduce, distribute, publicly display, edit, modify, and adapt the submission in connection with the Service in any form and media, and/or with reference to the Service, for a period of five (5) years. "
Even disregarding the royalty-free rights, I don't like the legal amount of room they have to completely change your material. Terms are always written to benefit the contest host, not you, the entrant. In this day and age, you really have to ask yourself if you truly need a middleman to give you a platform, something you can already give yourself for free.