Those accidents can be life changing, and often cause delays to services or dangerous levels of congestion during rush hour. I also suspect TfL are motivated to

Those accidents can be life changing, and often cause delays to services or dangerous levels of congestion during rush hour. I also suspect TfL are motivated to reduce liability - if they can show on CCTV somebody walking past a sign saying to take care, and then not taking care, TfL's coffers (i.e. the public's, it's not a private profit making entity), can be protected a little more from egregious legal claims.