it doesn't make sense to put only one price without tax

That's done because that's the norm in the USA, if you advertise a price without tax on Europe you will be legally bound to sell at that price.

americans have less purchasing power than before not than europeans

I didn't meant to imply that Americans have less purchasing power than Europeans (although they might, I don't know) but rather than it's common for companies to price things differently according to purchasing power and while the dollar did lose power that also means it's people lost purchasing power so it makes sense to keep it the same price instead of increasing it to compensate as otherwise they might loose sales.