@Reaver: As far is SR4 is concerned, it is a grey area that needed clarification, since the core book said you could cast spells on them, and that they could be attacked, but failed to mention what that actually means.
With SR5, you have the same issue, except Foci are some sort of weird exception, that are immune to everything on the astral plane. Mana Barriers can be attacked, Astral forms can be attacked, but foci are invincible.