I would like you to remember one thing: natural abilities (IE, not Su, Sp, or Ex) are duplicated by the Polymorph line of spells, as they are innate parts of that form.
Why do I want you to remember this? A Karsite's inability to cast spells is a natural ability; therefor, if you turn a spellcaster into a Karsite, they instantly lose the ability to cast spells.
Spellcasting isn't Na. Seriously, look at every definition of Na abilities ever.
Natural Ability: This term is a catch-all for just about anything a creature can do (or characteristic that it has) that is not extraordinary, spell-like, or supernatural. Natural abilities include most speed ratings (some very high speeds are not "natural," see the section on the alter self spell), mode of breathing (lungs, gills), natural armor and weaponry, general appearance, body type, and the presence or absence of the five basic senses (sight, hearing, touch, taste, pain). When polymorphing, you generally lose your own natural abilities and gain those of your assumed form.
Natural abilities are those abilities which are inherent to the physical form of the critter (like fly speeds for a critter that has wings, or claw attacks for a creature who has claws). This is backed up by the PHB and SRD.
Furthermore, NO special ability is natural. Here's the definition of a Special Ability:
Special Abilities
A special ability is either extraordinary, spell-like, or supernatural in nature.
Since Spells are a Special Attack, which is a subcategory of Special Ability, they have to be one of those three.
They are, however, Ex (I don't want to get into the full thing here, but they are), and those ARE lost if they're not from class levels. Note that while the ability to cast spells itself is not magical (you can tell because you still have the ability when you're in an Antimagic field, you just can't get the spells off without the Invoke Magic spell):
Extraordinary Ability: Extraordinary abilities are nonmagical. They are, however, not something that just anyone can do or even learn to do without natural talent or extensive training. Effects or areas that negate or disrupt magic have no effect on extraordinary abilities.
Extraordinary abilities often depend on particular physical adaptations that a creature has. Changing form often strips the recipient of some extraordinary abilities, but grants some extraordinary abilities that the assumed form has. In general, when you assume a new form, you lose any extraordinary special attacks and special qualities you have unless you get them from a character class. You usually gain any extraordinary special attacks your assumed form has, but not the assumed form's extraordinary special qualities. That's because most extraordinary special attacks are based off gross physical features (such a big, nasty claws and teeth) while extraordinary special qualities tend to be subtler and largely derived from a creature's essential nature.
So yes, polymorph does remove spellcasting, but let's not start calling them natural again (otherwise, you can Alter Self into something with casting and gain casting from that, since Alter Self grants Na abilities!).
JaronK