Stock based comp expense. If Amazon paid these employees cash instead of stock, Amazon's cash profits would have been lower. Instead they pay in stock, and employees pay the tax (which they're happy to do because the stock keeps going up). Furthermore if Amazon sold that stock in the public markets to fund those cash payments to employees, it wouldn't be revenue at all, and still not taxable.

From the article: "On average the thousands of staff who handle orders received about £3,000 per person last year. Senior staff will have taken home considerably more.

Amazon’s share price has surged 84% in the last two years. Last year the shares vested at an average stock price of $992, in 2016 it was $704 and in 2015 $467.

The payouts will have reduced Amazon’s tax bill because under UK tax law companies are required to deduct the vest value of the shares provided to employees."

Taxes are not paid on revenue. They pay sales tax on these goods sold in any case.