I'm not sure this is particularly unusual or even particularly aggregious. If Ubisoft had limited rights then it would be taking on too much liability to host the programme - and it'd be way too much work to individually negotiate the terms of each entrant's contract at this stage.

If it said something like "exclusive rights" then that'd be a major problem. The artist still owns the work and can make money from it, with this clause - they've just granted Ubisoft rights to it also. If you're planning on submitting your golden artistic nest egg then maybe hold back but for most people it isn't worth second thought when what is really on offer here is an opportunity.