Yeah, box office tracking would be less precise, but BO tracking isn't 100% accurate in the first place (there are rereleases, double features, etc. all the time that earn money but don't get tracked), and ticket sales are only one part of a film's moneymaking lifespan.
It may take a while to get off the ground, but it's become a big success in a short space of time over here, I'm sure the same would happen in the US. These schemes are growing every day.