If I am Astral Projecting, and I see some Metahumans, could I not cast Mana Spells at them? What is to keep a mage from frying a group of mundanes from the astral if they have Mana based attack spells?
To paraphrase the 2e rules (which I think explain this more coherently than any later editions I've read):
There are two scenarios under which someone casts a spell: they are on the physical plane, or they are on the Astral plane.
If they are on the physical, what happens is: as part of the process of casting the spell they synchronise their aura with the target's aura. The energy gathers around the caster, then travels between caster and target on the Astral plane, and then "grounds out" to the physical world through the target's aura. It is this grounding out, the transfer of shaped energy from the Astral to the physical, that causes the spell effects. When the target isn't something that is active on the astral plane, this grounding can only happen when there's physical target-physical caster symmetry because the caster needs to do this aura syncing to complete the circuit.
This is also why spells like Mana Bolt don't offer the target a defence roll (you can't dodge something that comes from inside you!) or consider things like armour (again, the spell bubbles up from inside your body -- can't armour against that!)
If the target is themselves astrally active -- a dual-natured being like a Materialised Spirit, an Astrally projecting or perceiving person, or similar -- then rules change. Now, that target is its own ground contact; it exists on both planes at once, so the caster no longer has to do the syncing aura thing. This means the caster can hide out on the Astral and lob spells from relative safety.
(This is only for Direct spells, like Mana Bolt. Indirect spells like Acid Stream are something different. There, the caster effectively casts the spell targeted at themselves ("I summon a stream of acid that will be directed over there") and the spell comes into existence right infront of them. Then, the magically created matter or energy travels from the caster to the target in the physical plane, like any other object. But this also means people who get hit by the acid get defence rolls and armour can help them.)