Thank you for the explanations.
However, if the power of magic comes from the weave, then potentially a wizard could cast a cleric spell if he knew the formalities because also the cleric uses power from the weave, only detouring through the deity he buckles under. Or not?
The argument "it's on a list" is not quite satisfying for me, although I work in the public service and deal with laws and rules a lot.

A question, however OT, as I'm playing a cleric currently and presumably will so after release: I would possibly prefer Sune as my deity (if the Internet did not fool me) but in her absence I'm a cleric of Tyr, and the hag said to me that she could tell me a lot about the crimes of the gods, which jarred a bit my hypocritical hubris as incarnation of law and order.

What about the gods?