Seems like a weak excuse... If you build a billing system which can calculate bills with low latency 99.9% of the time, but perhaps fails when there is a net split for example, then you can simply let the company write off the loss for that 0.1% of the time the billing system is down.
This seems the real answer. The question of whether the billing team can set accurate maximums is largely disjoint from the question of whether the engineering team shuts off service precisely at that moment.
