The same reason they accepted low offers in FR......to get in the door. The Cancer spreads to different organs.
The most deadly aspect of cancer is its ability to spread, or metastasize. Cancer cells initially group together to form a primary tumor(Ryanair). Once the tumor is formed, cells may begin to break off from this tumor and travel to other parts of the body(Other airlines). This process is metastasis. These cancer cells that travel through the body are capable of establishing new tumors in locations remote from the site of the original disease(Lowering of Ts&Cs).
TeaTowel, cancer can affect any being at any time. No one is safe.
You also make no mention of what you consider a cancer free airline.