What if a flight with 120 passengers is cancelled, and there's a UA flight with 8 empty seats? Now the policy needs to have a bullet-proof lawyer-approved tiebreaker system to decide which of those 120 passengers gets transferred to UA (otherwise the other 112 passengers might sue). What if one of the 8 "winning" passengers doesn't immediately present herself at the podium, now the policy needs to state exactly how long DL has to paige that person before giving away her spot, otherwise she might sue. What if there's a disabled passenger, or a family that has had particularly bad IRROPs luck on their journey, and DL wishes to help them out, but their hands are tied by this no-longer simple policy?

I think it's much better to empower employees to use their judgement rather than writing policies for irregular situations.