Bloomberg News on Monday was accused of violating its month-old vow not to investigate billionaire owner Michael Bloomberg or his 2020 Democratic presidential rivals after the outlet published an article criticizing Sens. Bernie Sanders and Elizabeth Warren for buying office supplies from Amazon—the retail behemoth both have criticized for low wages and poor working conditions.

The story, headlined "Sanders, Warren Campaigns Spend the Most on Amazon While Trashing It," was denounced as an absurd attack on two of the most vocal opponents of Bloomberg's candidacy.

"Less than a month after Bloomberg said a conflict of interest meant it wouldn't investigate 2020 Dem candidates, Bloomberg publishes a ridiculous hit piece on Mike Bloomberg's biggest critics in the race."

—David Sirota, Sanders speechwriter

"Less than a month after Bloomberg said a conflict of interest meant it wouldn't investigate 2020 Dem candidates, Bloomberg publishes a ridiculous hit piece on Mike Bloomberg's biggest critics in the race (and yes, the story is utterly ridiculous)," David Sirota, a speechwriter for the Sanders campaign, tweeted Monday.

Sirota was referring to Bloomberg News' pledge last month to not investigate Bloomberg or his 2020 Democratic primary opponents now that the former New York mayor is vying for the presidency.

"We will continue our tradition of not investigating Mike (and his family and foundation) and we will extend the same policy to his rivals in the Democratic primaries," John Micklethwait, Bloomberg's editor-in-chief, wrote in a memo to newsroom staffers on Nov. 24, the day Bloomberg announced his candidacy. "We cannot treat Mike's Democratic competitors differently from him."

Critics said Bloomberg's story on Monday was a flagrant violation of the outlet's vow to remain neutral in the primary process.

"Hit pieces on progressive candidates become questionable when run by a news outlet owned by one of their rivals," tweeted David Dayen, executive editor at The American Prospect. "This as much as anything shows why Bloomberg must sell Bloomberg News, the inability to do so is debilitating to democracy and a free press."

The article highlights critiques Sanders and Warren have leveled against Amazon, such as its avoidance of federal taxes, and weighs them against the amount of money their campaigns have spent on the company's website.

"So it may surprise voters to learn that the eight leading Democratic candidates and Trump have spent almost $600,000 on Amazon in the first nine months of 2019, mostly for office supplies, according to federal campaign records," wrote Bloomberg reporter Spencer Soper. "The stridency of the anti-Amazon rhetoric tends to correlate with the size of the outlay on its website."

The story was immediately and widely mocked on Twitter:

Sanders and Warren have aggressive plans to cut back on carbon emissions, yet they both inhale oxygen and exhale carbon dioxide https://t.co/7deBxRXxPY — Leah Greenberg (@Leahgreenb) December 24, 2019 Bernie Sanders rails against greed in the for-profit health care system. Yet when he had a heart attack, he went to a hospital. — Ryan Grim (@ryangrim) December 24, 2019 Warren, Sanders demand stronger clean-water regulations, but their campaign staff have taken thousands of individual showers in it and used it for hundreds of thousands of brushstrokes of teeth https://t.co/hAy23j7Srv — Joe Rospars (@rospars) December 23, 2019

Zephyr Teachout, a law professor at Fordham University and critic of corporate monopolies, tweeted that Bloomberg's article is "a non-story about two amazing candidates using an essential tool of modern society, notably the only two candidates who have hammered home the point that its very centrality requires it to be broken up and regulated."

"Amazon has too much power," said Teachout.