ISTM it is very much down to the individual, rider and org. The thing is, nobody 'owns' the roads so any claims to route copyright are a bit tenuous. Having said that it's right to respect the effort Orgs have put into setting up a route. My take is that if there is a listed perm available which fits in with my plans then I will ride it, even if it means some extra effort. If nothing else it adds interest to the outing riding somebody else's route. Elsewise I'll DIY the route.
For example, there are several 100km routes about 50km from home, some available as perms others not. Last year it suited my objectives to ride the routes as 200km DIYs from home (you cannot 'ECE' perms); where there were 100km perms available I mailed the Orgs and they were fine with it. This year I'd probably enter the 100km perm (if it is available) or not carry a Brevet at all.
Be assured most Orgs are more then happy to help riders, for example, by allowing them to start/finish between listed controls (from a convenient train station, home, whatever). As long as you hit the controls within the time limits the ride is still legal and most perms stipulate a min speed of 14.3km so tim limits are more generous than for a calendar. If in doubt, mail the Org.