I think the fantasist has the right to assert or imply that the laws of physics, history, animal husbandry, vegetable wifery or whatever (as we understand them) are wrong or incomplete. And I don't think the laws of the imaginary universe need to be absolutely explained, either: there's lots that science doesn't know about are world, and it's a cinch that some of what it thinks it knows is wrong. It's more a matter of being fair with the reader: if Gandalfclone used fireballs to rescue Frodolite from the evillish Orcoids, then why can't he use fireballs to light up the lightless caves of Suspiciouslysimilartomoriaville? There should be a reason. But I don't think that the mere throwing of fireballs have to accord with our understanding of physics, or even an alternate physics.

The Zelazny book I think of in this context is _Jack of Shadows_, where there is science and magic--both methods of approaching and manipulating a fundamentally ineffable reality.