The Old Law is an agreement between all major factions that if a reaper does not take on a servant or perform espionage, they are free to travel and reap souls without fear of being attacked or press-ganged.
The main reason why the Old Law is followed is because neither Abolish nor Vanguard has any particular inclination to break it. Vanguard thinks of themselves as the good guys and Abolish considers the reaping of souls to be a sacred duty not to be interfered with. However, should either side see a reason to violate the Old Law, it is also enforced by the Independents and Sai-hee, who would attack any faction that violated it, shifting the balance of power in favor of their enemies.
Aberration Controversy Edit
In the eyes of many, the creation of aberrations constitutes a violation of the Old Law, since aberrations are too undisciplined to be kept from killing neutral reapers, creating enough failures of discipline to constitute an attack. They may also be a violation of the spirit of the Old Law, which is intended to guarantee humans the chance to be reaped rather than abandoned or simply having their souls consumed. When aberrations first began to appear, they provoked a massive recruitment spike among the Vanguard from reapers seeking protection from predation by aberrations.
It is currently unknown whether the Independents agree or disagree with this. So far they do not seem to have made a formal statement on the subject or attacked Abolish, but this may be because they have decided that Aberrations are acceptable, they are still coming to a decision, or even because they are biding their time waiting for the perfect moment to strike.