Because it is a passive gain virtue (you don't have to activate it via the gump) and they havent put in a filter to turn the passive gain off. That is why the ability to access the virtue gump is removed when you are red - you still have the ability to gain passive virtues.
I know that it used to be possible to gain Compassion (another passive gain virtue) on a red by leading Bravehorn. Not sure if that was fixed or not - the bad part about that was that Compassion is not just a passive gain virtue, it is a passive use virtue as well.
Also, do reds gain justice when killing other reds? I will have to see what state my justice is in when I finish burning my counts back down below 5 ....