They cannot! (sort of). I believe in this case the USC was told that the consular officer, as well as the chain of command that reviewed the decision (7 people in total - whether they were rubber-stamping, I do not know) believed the hubby was in a gang.
The USC sent a letter from an "expert" that said that the hubby's tattoos had nothing to do w/ any gang (but as we know, expert advice is often available to those than can pay).
What was very illuminating was an amicus brief from some USCIS officers, that mentioned that these decisions are often totally arbitrary, the officers are learning on the job, that incredible and frivolous biases come into play (they had a story where, because the Iranian Revolutionary Guard has some interest in some - big - real estate company, a certain consular officer was rejecting everybody and anybody from Iran who was in the real estate business. Just in case... What kind of explanation can one be offered in these circumstances? - this is pure insanity).
However, given the administrative constraints (time, workload etc) the only honest explanation I can think of is "we could not, in the 1/2 hour allotted to the case, convince ourselves that all was kosher". Everything else requires discussion/investigation/... - things there are no resources for (if a new law makes such work compulsory... I can see wait times exploding).