It's the power of defaults that make them easy to abuse. To the point of that study, Richard Thaler who has studied the subject extensively argues more easy mandated choice (i.e. you have to make a choice one way or another when you sign up for a drivers license). Opt-out is a sufficiently powerful default that it's reasonable to assume that many never made an actual reasoned decision to do so. For trivial matters, it may not matter much, but in the case of something like organ donation families can and have argued successfully that the deceased never actually made a choice in the case of opt-out.
