I thought about that possibility as well! I really liked Joyce a lot and just like the characters, I had a problem accepting the situation.
But, as much as I'd like to believe that, I still doubt that magic had anything to do with Joyce's fate. As sad as it was, it was really important for the Scoobies (especially Buffy) and their journey to adulthood, if you will. They knew how to stop apocalypses, fight evil and all, but, still couldn't accept the basic facts of life, as we can see from their reactions and how they tried to blame it on something supernatural. But, things, sometimes, happen for no apparent reason! That's life! And, maybe, this is why life is both beautiful and really, really scary! Magic, or no magic.
Just my two cents...