I assume you mean the online Apple Store. Yes, they will charge you state sales tax IF Apple has a "presence" in your state (assuming your state is Iowa.)
There are at least 5 Apple stores near Iowa City alone.
This is the same for any online purchase. Now if you have ordered your iMac from say "J&R Music World", it would have be a tax free purchase. However, then there may or may not have been a shipping charge.