Poor Nigella Lawson. Not only has suffered a messy, public divorce from business tycoon Charles Saatchi over the past year, but now she can't fly to the U.S.

The U.K. celebrity chef was prevented from boarding a flight to Los Angeles on Sunday because of a court confession during her divorce about her previous drug use. Lawson now likely will have to undergo a medical examination, including blood tests to screen for illegal drugs, if she wants to obtain a travel visa to the United States, a British lawyer told the Telegraph.

So forgive us for asking, but why is Canadian singer Justin Bieber still allowed into this country? Bieber has spent most of this past year fending off drug use allegations and criminal investigations; yet, he still maintains residences in the United States.

FILE - In this Wednesday, Dec. 4, 2013 file photo, British chef Nigella Lawson arrives at Isleworth Crown Court in London. The U.S. Embassy in London said
FILE - In this Wednesday, Dec. 4, 2013 file photo, British chef Nigella Lawson arrives at Isleworth Crown Court in London. The U.S. Embassy in London said on Thursday, April 3, 2014 that Lawson was denied permission to board a flight to the United States on the weekend. The embassy did not disclose the reason for refusing Lawson. (AP Photo/Sang Tan, File) ( Sang Tan )

For starters, Bieber has never admitted to drug use -- Lawson testified last year that she had snorted cocaine and smoked marijuana in front of her children. Bieber's current legal troubles also aren't related to illegal drugs; he faces DUI charges in Miami, where after an arrest for alleged drag racing, Bieber reportedly told police he had consumed alcohol and prescription medicine. He also is being investigated in Los Angeles for allegedly throwing eggs at his neighbor's home; the investigation led to a search of Bieber's home, where marijuana was discovered but the singer was never charged.

In Lawson's case, her admitted past drug use during a trial last year raised a red flag. According to U.S. border officials, they can bar entry to anyone who admits to "committing acts which constitute the essential elements" of the crime, including abuse of a controlled substance.

Susan McFadden, a U.S. attorney based in London, told the Telegraph that if during a medical examination,"the doctor finds the applicant is a drug abuser than that person is ineligible for a visa. This can be set aside only if the consular official recommends that the ineligibility for a visa should be waived."

McFadden noted that such waivers are decided by the U.S. Department of Homeland Security and can take six to seven months to process.

Such bans have prevented celebrities from entering the United States in the past. In 2008, singer Amy Winehouse was not allowed to attend the Grammy Awards. Singer Lily Allen lost her work visa in 2007. And the U.S. Department of Homeland Security placed singer Yusuf Islam on a no-fly list in 2004.

But apparently, U.S. officials also can choose to turn a blind eye. Last month, Toronto Mayor Rob Ford -- who has admitted to smoking crack cocaine -- was able to travel to Los Angeles for an appearance on Jimmy Kimmel's late-night show.

The double standard had celebrity chef Anthony Bourdain in an uproar Thursday. He tweeted, "Toronto mayor, Rob Ford? Welcome to the USA. Nigella Lawson? No. REALLY? Absolutely appalling misuse of our system. And by whom? How?"

Joel Sandaluk, a Toronto immigration lawyer, offered his theory on that to the Toronto Star. "It's hard to imagine a U.S. border officer who is unfamiliar with Rob Ford -- I mean, he's introduced in the American media as Toronto's crack-smoking mayor," Sandaluk said. "All I can imagine is the officer who he encountered at the airport simply followed a 'don't ask, don't tell' approach to it."