Well obviously they won't intentionally roster illegally, but legal doesn't always mean safe, and when it comes to fatigue some pilots may have more trouble with a roster than others.
Take the example of a company that only does back of the clock night freight. You might find one pilot who can't sleep during the day after duties 1 and 2 but by duties 3 and 4 he's so knackered that he'll sleep through anything. Another pilot on the same roster may not be able to sleep during the day ever, being perpetually tired but not sleepy. A third pilot may not have a problem with any of it; gets home at 7am goes to bed, sleeps for 6-8 hours and gets up. The first pilot gets by and may have to call in "sick" once in a while when the sleep cycle just hasn't worked for him, the second pilot is in the wrong job, while the rules are written for the third pilot.