I've noticed that if you pull back the lining of the trunk of most cars you see the back seat, there usually isn't anything solid between.
Even thous of us with trucks should still be careful as whatever is in the bed could still come over the top of the bed and through the rear window of the truck.
I'm going to make a junk yard trip one of these days and pick up some seat belts to make some cargo straps for under the seats. to help hold some of my gear in better. Figure if I find seat belts material in a matching color as my interior it will blend in nicely.