Why offer the procedure without drugs? Many of the vets were unable to travel to a facility that would perform the procedure under sedation. Others did not have anyone to take them home after the procedure (this is necessary when "twilight sleep" is used, which makes patients groggy after a colonoscopy). In addition, the Center had a shortage of nurses.
To improve upon their procedure, doctors used water instead of air to expand the bowel. This was especially helpful in cases where bowel prep was poor, which can result in an incomplete test.
I've had several colonoscopies (when I still had a colon!) and I'm not sure I could do it unsedated. Bravo to those brave souls who not only took an uncomfortable test that could save their lives, but did it sober.
Medpage Today. "Water Infusion Boosts Performance of Unsedated Colonoscopy." 22 May 2008.