Forgive me if I am misunderstanding something here:
the report says
it says that some drivers do not think they would be caught using a mobile device while driving, not that they think it is okay. If the survey results showed that 40% of Britons thought they could get away with murder, that doesn't mean that 40% 'think it's okay' to murder someone.
What can be understood from this is an understanding of how effective drivers think enforcement of policies in the workplace is. The survey (that is quoted in the RAIB report) does not ask whether drivers think it is okay. Similarly, the same can be applied for the latter, 18% of drivers think using a phone does not affect performance. It does not say that they would.