[crazygringo]

I mean, what's the rationale in the US for a business being taxed on their profits, and also having to pass along the sales tax they've collected?

It's just two forms of taxation. Sales tax/VAT is a fixed proportion of sales, and then you also pay tax on profit that's left.

You might as well ask why people pay income tax when they make money and then have to pay sales tax/VAT again when they spend it!

Of course, answering that is complicated, and there are a lot of factors. But the main one is basically that governments like to tax "everything", so that people/goods/services that might wind up evading one tax wind up paying another. Sales tax makes sure governments get revenue even when businesses make no profit, taxing profits makes sure governments get more revenue when businesses make more money.