I think I could have sat in the B24 top turret, but I didnt because in the preflight briefing we were told that the only thing you will see is sky, which gets boring.
The B24 nose turret was off limits because the turret had some fragile parts to it. (even though it was nearly identicle to the rear turret which you could get in.)
I cant remember why the B17 tail and upper turrets were off limits. It could be that if they hit some turbulence, you could easily bang your head against the guns and get hurt.