I really don't see that as a significant contribution to their winning. All of the winners were well painted even from a single angle. Does it provide some help, possibly, but by the same token so does ones skill in taking the photos to begin with.
I would honestly have more cause for concern that photo retouching in the hands of an expert photographer would have far more implications for unfair representation than simply adding a couple of images together. (In fact, I limit my own adjustments to color balancing, exposure tweaking, and slight sharpening when I submit my images for the competition.)
Despite that, I think you should also remember that Canucks Fan has won the ship competition for the last two years (nearly won it three years ago too) and the 2008 winners entry was a single angle of the ship that didn't even show the entire thing.
Doing a cursory glance at the heat winners too, it seems that there is a fair distribution between composite photos and single angles winning as well.
If you are concerned that photoshop provides an unfair advantage, you could always look into using the Gimp. It has a very similar feature set to photoshop and is open source so you can grab it for free.
