My guess is because the menu is not visible, there is no way to display a sub menu{In other words Adobe did not make that provision in the code}. Alt-space is being used by photoshop so the OS can not use it.
However all the ctrl-keyboard shortcuts still work is is really considered the norm.
In normal mode (menu is visible, take a look just to the right of the sub menu option and you will see the keyboard short cut). Also in the Edit>keyboardshortcuts or {Alt-shift-ctrl-K} you will find a Summarize button, this will generate an html file containing all the keyboard shortcuts for your copy of photoshop. Meaning it includes any changes you have made.
Also take note of what happens when you use the rightmouse button and shift-right mouse button when the cursor is over the document.
I hope this helps...
