Exactly when is the "theft" happening here? You're not claiming the NM, so they can't be taking something that belongs to you. The issue at hand seems to be that you're slow and you probably don't understand some notorious monster spawn mechanics.

If this change were implemented, what's to stop people from logging in or entering an area 15 minutes prior to spawn and claiming when it spawns? Nothing. You'd be back on this forum making more suggestions that either aren't very well thought and help no one.