My 06 Mazdaspeed6 does the same thing. I think the reason for this is to keep cargo in the trunk secure from theft. In my MS6, I can lock the trunk so that it cannot open using the "door unlock" button inside the car. Effectively, if a would be robber breaks the front window, s/he can steal anything inside the passenger compartment, but there is no way to access the trunk without the key. Several other manufacturers have a "trunk open" button near the drivers seat that can be locked using the key as well (1996 Crown Vic comes to mind).
As crazy as it sounds, this happened to a family member after he placed his wallet and cell phone in the trunk of his car when he went to the beach. A robber broke the window, crawled through the fold-down backseat, and made off with his personal items.
I've seen older Japanese cars have a key lock on the inside latch to fold down the rear seat. Also, many cars have a "defeat" switch usually in the glovebox to disable the trunk button from the passenger area. The idea is that you also lock the glovebox, and now the trunk is secure.
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As crazy as it sounds, this happened to a family member after he placed his wallet and cell phone in the trunk of his car when he went to the beach. A robber broke the window, crawled through the fold-down backseat, and made off with his personal items.