Ok, if your attic is easy to get to, ie., there is easy access, a ladder, steps, whatever, I can see why you might be inclined to install a smoke detector top side..
However.. Picture this if you will..
I wake up at 7AM on Saturday to an insane screeching noise.. I cannot tell where it is coming from.. It is like an annoying ringing in my ears. What could it be? It's even louder as I start heading up the stairs. I go outside and find the ear piercing squeal grows even more obnoxious.
Then and only then does it occur to me that the noise coming from the attic vent on the side of my house contains a smoke detector.
What the fuck? We've lived here over 5 years now and never heard a peep... So some idiot put a smoke detector in our attic.. It's not an easy access attic.
To kill this insane noise maker I had to employ the use of a long broom handle and my neighbors on loan step ladder... After beating that bad boy for several minutes with the broom handle I was finally able to dislodge the battery. No more noise...
I ask again, who in the fuck puts a smoke detector in an attic?