maybe they needed the cameras there for scientific purposes.
You know, to observe how its done and learn important science stuff.
If he knew about the camera, he'd either not donate, or play it up for the camera porno style.
And we all know, that shiz ain't natural.

Plus maybe they want to be sure he was donating his own fresh specimen, not a container of something his dog "donated", just so he could get an easy $100. (not that getting your dog to hand it over is any easier than doing it yourself, but hey there are some sickos out there).