I'm actually giving Cameron the benefit of the doubt.
It seems to me that he asked Coulson if there were any general issues he should be aware of, Coulson said no. I'd imagine Cameron asked specifically if there were any issues relating to phone hacking etc. I imagine Coulson said no.
Cameron saw a man who probably swore on his word that he had not been involved in wrong-doing but who had done the decent thing and resigned.
Cameron's background checks probably found no other faults, and so he was employed.
If the above scenario is anywhere near correct then I find no fault on Cameron's part, other than being trusting, which is no great fault.
In that case, Coulson comes off very badly.
If this was not the scenario then I think Cameron may have questions to answer