|James Jacobs Creative Director|
Actually... the fact that waters of Lamashtu and a few other spells from Gods and Magic have made their way into the Inner Sea World Guide is kind of a stealth revision to that rule from Gods and Magic. You CAN cast a spell like this if you don't worship Lamashtu, just as you can cast shield of the dawnflower[i] if you don't worship Sarenrae, or [i]Gorum's armor if you don't worship Gorum.
Casting/preparing those spells, though, is akin to offering prayers to that deity. And that can have alignment and faith repercussions... but since those repercussions depend ENTIRELY on the relationship between the two deities in question, we leave those rulings to the GM.
The simple solution is, of course, to say that you can't cast a spell from a specific deity unless you worship that deity. Essentially, you as the GM are roleplaying the god and you can say to the player, effectively, "That spell's not allowed to you, and don't ask again or I shall smite thee!"
A more interesting solution would be to say that you COULD cast spells outside of your deity's command... but the cleric should be almost as concerned by having such spells granted as anyone else. Why WOULD Calistria allow one of her clerics to cast a spell tied to Lamashtu? Does Calistria have a thing for Lamashtu? Does she owe Lamashtu a favor? Is it even Calistria you're talking to?