I don't think I would have chosen that approach; to my lights, anyone deterred from finishing due to the bombs had as real a marathon experience as those who did cross the line. (Note, perhaps irrelevantly, that the original Boston Marathon route was 25 miles. http://www.boston.com/
zope_homepage/sports/marathon_archive/history/1897.shtml. I do not believe anybody was pulled from the course before making it that far.)
This will have a substantial impact on the registration process next year, too. Either charity slots will not be available or the qualifier slots will be dramatically reduced. Because I doubt many who are offered the chance to run next year will say no. If the marathon in Boston was not iconic before, it surely is now.