Referring to the 'Estimate Shipping and Tax' section on Shopping Cart page - Magento EE 188.8.131.52, by default the 'Country', 'State/Province' and 'Zip/Postal Code' are not required.
This block is in the following template frontend\enterprise\default\template\checkout/cart/shipping.phtml
As in the code snippet below, the 'required' (CSS) class is added only if $this->isZipCodeRequired() is set to true.
<label for="postcode"<?php if ($this->isZipCodeRequired()) echo ' class="required"' ?>> <?php if ($this->isZipCodeRequired()) echo '<em>*</em>' ?> <?php echo $this->__('Zip/Postal Code') ?></label>
My question is: where is Magento checking for these values and can they be set in Magento Admin?
Note that the field "Postal Code is Optional for the following countries" in Magento Admin may not be an answer for this as, the above code snippet doesn't depend on the country.