According to the link moriador provided, unless DAZ is providing a physical backup (which, IIRC, they stopped doing a few years ago), then regardless of whether they have an office in California or not, purchases of digital goods through their store are not subject to taxation.

I don't suppose anyone has asked DAZ about the why's and wherefore's of this tax?

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If you're joking that's just cruel, but if you're being sarcastic, that's even worse.