In the US there are taxes at the time of sale. These taxes change depending on where you are making the purchase, where you live, who you are, and what you are buying.
The POS system knows how to calculate the cost of each item, why can't they just out that on the sticker instead,.
However if the sale is online and the company has a presence in multiple states they have to charge based on the user, which they cant do until they know who the user is.
Edit: Just to be clear I wish they would put the final price. Here is a picture I found of Pat's King of Steaks in Philadelphia, they have price tax and total for each item.
https://media-cdn.tripadvisor.com/media/photo-s/03/2c/33/c2/...