Initially, I had the exact same question. I came upon this explanation on the ETS website (water purification section, Micropur review) that set my mind at ease:

"We also inquired about the safety of using the tablet at higher concentrations, in other words, in less than a liter/quart of water, as might be the situation with an improvised water container in a survival situation. We were told that this could be acceptable for short term use. A higher concentration (less water) may result in unpleasant taste or odor. If someone has an adverse reaction to chlorine dioxide in general (which is a rare condition), this risk would increase (similar to the case with those having adverse iodine reactions). However, in an emergency situation, no negative health issues would be expected."

http://www.equipped.org/watrfood.htm#WaterPurification