This is a limitation to the way the paypal standard system works.
The paypal standard system does not allow for passing the shipping information.
the work around is to use the checkout wizard to create the checkout pages, the checkout page will collect shipping and billing information, and pass it to the confirm page.
I eCart, you can set the tax Calculation to charge based on the selected shipping state, this can be set while running the wizard, the confirm page will then pass the transaction to the paypal site including the tax rate.