Can i change the default Mac App Store install folder?
Regarding Mac OS X File System Overview, it seems Apple enforcing the standard compliance for developer to follow their rules on application folder which are within /Applications or ~/Applications. The concern is to comply the way Mac App Store deliver the updates.

if you not comfort enough to use default folder, yes you are still able to move the application which you had download from Mac App Store to another folder with two caveats:


You will be required to enter an administrative password to move the
app from the /Applications folder.
If an update appears in the MAS for an installed app that has been moved you will get an error message about having apps installed
    from another account. To update the app you will have to Delete
the
    app entirely and then install the updated app or Move the app back to the /Applications folder