I am a Mystic Theurge. This makes me, at least to a degree, a divine caster. I sleep for 8 hours and am now rested. I light a block of incense of meditation and pray for the next 8 hours. At this time I prepare spells. They are maximized for the next 24 hours, because they are MY spells. Are my arcane spells mine? Or, do they belong to a different me?
The description of the item gives three things. First, who can use this item? A divine caster. Second, how is the item used? By praying for 8 hours within the specified radius of the burning incense. Third, what effect does this have? All spells that I prepare are maximized. Not all divine, just all. Just because all of these conditions are stated in the same sentence does not change this.
I agree that the intent is obviously that the item divine spells only, but the description is not that specific. If you like the intent, modify the item to fit. If you like the other, that's RAW.
AS to whether or not I feel the item is balanced when it includes arcane magic too, the answer is YES. A split caster will not be able to provide the same level of power that a straight caster can (and, yes, I am aware of the practiced spellcaster feat, which is an additional cost). The use of the item requires foresight and planning to be effective, and only works on healing and/or damaging spells. That said, when the character can pull this off, he deserves the full effect for his ~5000gp. IMHO.