The reason sales prices are quoted without tax in North America is because there's a million different permutations governing what the sales tax is going to be in different situations (who's buying what, from whom, where, when), and this changes all the time.
Local governments can shift their taxes at any time to encourage or discourage certain things. I'd argue this flexibility is very good, but the downside is calculating sales tax in the US is complex enough that there are companies dedicated to it.
When I'm standing in a store, they absolutely know what the tax is going to be at the register.
Online, I admit it's harder... But they still need to figure it out at checkout. I would be fine without a disclaimer that until the shopper's location is specified, the final price can't be calculated, but if they're logged in and it's set, it should show it.
Everyone quotes pre-tax pricing because it tricks people into thinking the prices are lower than they are, just like $9.99 feels cheaper than $10.00 when you're making a quick judgement, even though that penny is inconsequential. And it's why gas stations add the additional 9 behind it, making the actual difference even more pointless.
Well, yeah, but you can also see how the key piece of information a company discloses is the pre-tax price.
To show each customer the right post-tax price, you'd have to know where they live and where they will buy your product. It would be impossible to advertise a price en masse on your website / in a brochure.
Local governments can shift their taxes at any time to encourage or discourage certain things. I'd argue this flexibility is very good, but the downside is calculating sales tax in the US is complex enough that there are companies dedicated to it.
So everyone quotes pre-tax pricing.