MonoTouch doesn't currently support position independent code, when we AOT the managed code we generate position dependent code.
We may fix this in the future, but right now it's not a priority.
Update
Apple has just started warning about non-PIE enabled executables when publishing to the App Store. This will obviously shift our priorities.
Update 2
We have a potential fix which is going through testing / QA (but no firm release date yet).
Update 3 (2015/03/21)
This was fixed a long time ago.