It's not even that they don't understand fallacies.  Usually adult people are pretty good at spotting flawed reasoning even they don't know the formal definitions of fallacies.  It's just that when it's to do with something you either want to believe or have already convinced yourself of that I think you're susceptible to falling for these flawed justifications and defenses.  I think social pressure is also a huge part of it.  You're more likely to accept something if it's constantly being presented as the socially acceptable, or if you prefer, asserted as the morally correct attitude or belief.