Background: Anthony Neil Wedgwood Benn (3 April 1925 - 14 March 2014), originally known as Anthony Wedgwood Benn, but later as Tony Benn, was a British politician, writer, and diarist. He was a Member of Parliament (MP) for 47 years between the 1950 and 2001 general elections and a Cabinet minister in the Labour governments of Harold Wilson and James Callaghan in the 1960s and 1970s. Originally a moderate, he was identified as being on the party's hard left from the early 1980s, and was widely seen as a key proponent of democratic socialism within the party. Benn inherited a peerage on his father's death (as 2nd Viscount Stansgate), which prevented his continuing as an MP.
Context: In 1997, the Labour Party under Tony Blair won the election. Despite later calling Labour under Tony Blair "the idea of a Conservative group who had taken over Labour" and saying "[Blair] set up a new political party, New Labour", Benn's political diaries Free at Last show that Benn was initially somewhat sympathetic to Blair, welcoming a change of government. Benn supported the introduction of the national minimum wage, and welcomed the progress towards peace and security in Northern Ireland (particularly under Mo Mowlam). He was supportive of the extra public money given to public services in the New Labour years but believed it to be under the guise of privatisation. Overall, his concluding judgement on New Labour is highly critical; he describes its evolution as a way of retaining office by abandoning socialism and distancing the party from the trade union movement, adopting a presidentialist style of politics, overriding the concept of the collective ministerial responsibility by reducing the power of the Cabinet, eliminated any effective influence from the annual conference of the Labour Party and "hinged its foreign policy on support for one of the worst presidents in US history".  Benn strongly objected to the "immoral" bombing of Iraq in December 1998, saying: "Aren't Arabs terrified? Aren't Iraqis terrified? Don't Arab and Iraqi women weep when their children die? Does bombing strengthen their determination? ... Every Member of Parliament tonight who votes for the government motion will be consciously and deliberately accepting the responsibility for the deaths of innocent people if the war begins, as I fear it will."  Several months prior to his retirement, Benn was a signatory to a letter, alongside Niki Adams (Legal Action for Women), Ian Macdonald QC, Gareth Peirce, and other legal professionals, that was published in The Guardian newspaper on 22 February 2001 "condemning" raids of more than 50 brothels in the central London area of Soho. At the time, a police spokesman said: "As far as we know, this is the biggest simultaneous crackdown on brothels and prostitution in this country in recent times", the arrest of 28 people in an operation that involved around 110 police officers. The letter read:  In the name of "protecting" women from trafficking, about 40 women, including a woman from Iraq, were arrested, detained and in some cases summarily removed from Britain. If any of these women have been trafficked ... they deserve protection and resources, not punishment by expulsion. ... Having forced women into destitution, the government first criminalised those who begged. Now it is trying to use prostitution as a way to make deportation of the vulnerable more acceptable. We will not allow such injustice to go unchallenged.
Question: why were they arrested?

Answer:
is trying to use prostitution as a way to make deportation of the vulnerable more acceptable.