Clerics could be powered by those who favor his alignment... I mean, they still use a Holy Symbol, and Divine Focus. Also their spell are considered Divine... which wouldn't be the case if they weren't powered by a divine force.
Seeing the things this way... Druids would actually be using the power of nature as divinity... which could be seen as actually weakening it?
What powers a deity? I may be thinking about a different lore, but I think it is the amount of worshippers it has? I am not really sure how following a Good ideal would be considered praise to the gods of Good... but it is in itself a manifestation of Good... so, in a way, it is inserting more Good in the universe... by consuming the life force of the creature that follows the ideal, since every second of life is a second of life force consumed.
That could apply as well to Evil... but Neutral? isn't Neutral supposed to be zero sum? Well... if you are not following neither a Good nor an Evil ideal, the energy consumed of your life force has to go somewhere... and that could very well be Neutral. There are Neutral gods, right?
Thus... the capacity to cast a divine spell, first transfers the life force to a higher power, be it a specific god or a cosmic force powered by the alignment of the ideal, and then comes back to the caster, making it "divine" in nature. This seems suboptimal, since energy transfer usually includes a loss of energy that is disipated, and in this case it would be two-fold, when it could have powered the caster directly with lower energy loss... but there go the fighters, barbarians, and other non-casters, that cooperate to the same cosmic force, without using it. Also, not every second of a caster's life is spellcasting.
Now... Arcane... life force converteg in magic, stored within the caster? since it has to come from the inside, that explains why arcane has a chance of failure when using armor, when divine doesn't have that limitation, even when it is the SAME spell that is being cast.
Also, the higher levels, both Arcane and Divine, can cast higher level magic not only because they can channel it better, but because a bigger portion of their life force that has been converted for that purpose.
I mean, maybe.