Why didn't they just say "Yes, we knew that the train would be crowded so we went along to get some good photographs to illustrate that and force Branson and others to make some changes" ?
Stunts make news, which can be a lever to cause improvements, but the public is inclined to be less sympathetic when they find out that it was set up.
In 1885 William Stead, a reporter/investigator, wrote a series of articles for the Pall Mall Gazette on child prostitution, entitled "The Maiden Tribute of Modern Babylon". He uncovered horrifying stories of the exploitation of young girls (and boys) at a time when the age of consent was 13, usually without actually asking that consent.
To illustrate his articles, he found out how to "acquire" a girl of 13, and negotiated to "buy" one from her parents. He took great care that the girl was chaperoned at all times and intimately examined to prove that she had not been interfered with while in his charge. The stunt led to the raising of the age of consent to 16 in 1885, and to more public awareness of the evils of the trade, but Stead and several of his accomplices were later brought to trial as a result of the unlawful investigative methods they used. Stead himself served three months in prison. (her mother denied on the sale and said that he had claimed to be hiring a kitchen-maid, as if any gentleman would pay over cash to the mother of a potential skivvy)