Maybe they merely point out that merchants cannot hide their identity and hence be identified and taxed. Not that it would be deducted automatically.
Providing anonymity for customers AND collecting sales tax (in the US) are two goals that conflict with each other. Maybe there are ways to resolve the conflict, but that's far from obvious.
I'm not talking about whether it's automatic or whether GNU Taler makes it easier. I'm talking about whether making the seller info public is enough to comply with tax laws.