Can people turn off your phone when in lost mode?

I have lost my previous iPhones in a few circumstances but have never been able to recover it. Recently I have again lost my iPhone 6s which I have not backed up in awhile so I have important data and photos etc which I really wish I could get back. I like the idea of Find My iPhone app which helped me put my phone in lost mode and track its location in the past. However I was able to track my phone for a period of time and tried to reach its location to recover before it became offline and was probably turned off. I wonder shouldn't this feature also restrict any person to manually turn off the phone at least without a passcode or until lost mode is removed or the phone recovered...because if it ends up in the wrong hands, the person can simply turn it off and that will render the find my iphone function almost useless. Maybe the phone will remain locked once turned back on or requires activation lock, but it doesn't stop anyone to just eventually sell the lost phone and remove parts for resale and use. If there is such a feature, I think it would really increase the probability of finding a lost phone and which would be the real purpose of the Find My iPhone app and increase consumer confidence in having a secure phone.