I never understood the obsession with "unauthorized leave" for contractors or if a large part of your pay is based on work done, sample sector pay. As long as you eventually fly close to your maximum allowed hours an airline should just build in buffers to take account for the weakness of human physiology.
I suspect it is an unwillingness to see the difference between different groups of employees; If one can come in and sit behind a desk with the flue, another can fly a plane even if under the weather. If one goup of employees are prone to goofing off, another with more responsible jobs and backgrounds are likely to do the same given halve the chance.