Hello Mark,
Yes you are right that the torque applied to the nuts is the same as the torque applied to a head bolt. However, the nut is made of the same material as the stud. Steel is far better at resisting spalling, or friction caused crystallization of the thread material in the heads.When a steel bolt is tightened into aluminum threads, there is a sideways force applied to those threads and even with lubricant. There is a possibility that a previous mechanic has used unlubricated bolts, causing damage to those threads.
Anyone who has had spark plugs seize up in a head will have dealt with spalling. This issue is often solved by using a small amount of lubricant on the plug's threads or threaded inserts. That said, I would still prefer studs to bolts when attaching anything to aluminum.
Thanks for your comment,
Larry