I'd definitely prefer it if stores would just go ahead and factor the sales tax into their prices, but the reason they don't is probably so they look cheaper than they are. Like, I've seen things advertised for "under $70" that are priced at $69.99, but unless you buy them in a state without sales tax, that's actually going to work out to considerably more than $70. It's a trick, really; $69 LOOKS quite a bit cheaper than $70 simply because there's a 6 in the front.

The Golden Rule is usually a good guideline, although there are probably situations where it doesn't quite work out, because not everyone necessarily WANTS to be treated the same way. To give an extreme example, what if you're a masochist? I guess it works better if you use it as a general attitude than something totally literal.