There's nothing fundamentally simple when you realize that the POS system is just one of the many front-ends for the store's inventory management, sales and cost reporting, barcoding and label printing, purchasing and tax accounting software. If you ever have a chance to help open a new retail store and actually see all the moving parts, you might want to take it, as you'll learn why POS software isn't as simple as it looks, and why the affordable ones are limited to small single-store establishments in specific industries.
Just handling taxes in a piece of software meant to be used in multiple locations can be hideously complex, and that's just 1% of what the system is responsible for. Remember that not only do the tax rates differ from one half of a town to the next, but what items are taxable do too.
You make a good point. I guess in the general case it's "not simple". I've built a number of inventory and ERP systems, so it's kind of my wheelhouse now. But yes, such systems aren't necessarily trivial without a rock solid design. I can build a goodish ERP system now because I have experience, but to someone just entering the field it has a lot of good looking corners that turn out to be awful decisions.
Just handling taxes in a piece of software meant to be used in multiple locations can be hideously complex, and that's just 1% of what the system is responsible for. Remember that not only do the tax rates differ from one half of a town to the next, but what items are taxable do too.