I was recently approached by a company that offered to sponsor a photo contest on my site with some nice prizes but stated that as part of the contest, they would retain the non-exclusive rights to use the winning image (but not other entries) in an advertisement for their own product if they choose. It will not be distributed to others, or employed for any other use.
If I were to go ahead and have the contest, making the above conditions very clear and prominent, would this constitute a "rights grab"? Would it be unethical and/or controversial?