The AR is designed so the safety cannot be engaged unless the hammer is cocked. This gives the shooter a way to determine what position the hammer is in at any time. The same is true with the safety of the M14 and M1 Garand.

The AR that is an exception is the HK. The safety can be engaged regardless of which position the hammer is in. I also know of two aftermarket triggers that allow the safety to be engaged regardless of the hammer position, the Franklin Armory Binary trigger and the Fostech Echo.

I don't know of any AR that requires the hammer to be cocked to open the upper. Some do need the hammer cocked to close it. All of my ARs can be closed with the hammer down except except the MP-10 with a Geiselle. I haven't looked into it, but I think it's due to the firing pin spring. It holds the firing pin back and the back of the firing pin hits the top of the hammer.

Sometimes the carrier slides out just a bit and smacks the top of the buffer and leaves little dings around the edge. The moral of the story is, don't just slam the upper back into place when closing it