The problem, though, is that it often comes up with 0, so even re-wording to something like "this flight might earn/will earn approximately/at least 0 miles" doesn't change anything.

But really, there is no excuse for not being able to fix it. The booking engine knows the origin and destination, and the fare classes; if it can look up the correct fares, there is absolutely no reason why it can't be fixed to look up the correct mileage, as well.