It's tax-related, but also because every state, city, and county can add their own sales taxes, so the only way to advertise prices for a national chain is by excluding sales taxes. As a result most businesses, even just local ones, follow suit. And the taxes can change after any government budget meeting or vote. Plus, many retailers attempt to keep pace with or beat inflation, so prices tend to slowly rise, by pennies at a time.

> There's no 'leave a penny/take a penny' tray in Australian retail culture, for example.

There wasn't one when I was a boy in the US, at least not that I remember. I think that they started to be a thing around the time I graduated college, and of course they're now everywhere. Just today I paid for some coffee and got back an integer number of dollars because the clerk made up the cost. Of course, it was in her interest, because I throw change or a dollar into the tip jar when the tab is small enough that the tip is less than a dollar.

I down-modded you because references to when you were a boy and when you were in college without actual dates are meaningless to everyone else.

I really don't think that's worthy of a downvote, though. Being deliberately obscure with dates and ages can even be interesting sometimes.