The trouble is that Americans seem to be onboard with not adding taxes, and so that's your slippery slope right there. In Europe regulators are like "If the headline price is €10 but actually nobody pays €10, that headline price is wrong and must be fixed" and they're including taxes in that.

In IT there was a UK battle years back when magazines were still a big deal about whether you could advertise prices without VAT. All these companies charge retail customers VAT of course, but in effect the way VAT works the price is without VAT if you are yourself a business that claims VAT refunds. So the argument in some of these magazines was, look, you can buy this magazine as a hobbyist, you can buy this Western Digital 200MB hard disk from an advertiser as a consumer, and you'd pay VAT. But actually they advertise in our magazine because many readers work in IT, so while they might get 10 hobbyist orders for that 200MB disk, they also get one or two business orders for a hundred drives, and those customers don't pay VAT so why must we show VAT prices ?