Most sales tax laws are also tied to use tax... if they can't change sales tax (because the seller is out of state), they charge a use tax that's equivalent (this is has been unenforced in many places, Texas included.. but some states have been pursuing use tax on internet sales- and that's exactly what is the case that was heard before the SCOTUS) The constitution forbids states from levying an import or export tariff, which is what it would be if Texas taxed goods leaving the state bound for another state. Sales/use taxes by states are not verboten if they apply to both intrastate and interstate commerce.
If the product ships to California, it's not staying within Texas borders. Simpler to keep it in state? sure... but most politicians never met a tax they didn't like.
Thanks, that makes sense. I keep going back to the "use" part of "sales and use". It's there and it seems inherently constitutional. That it's difficult to collect is too damned bad.