I think (I obviously don't know) that Trump says things like, "I have confidence in him" as a way to maintain confidence in the system as a whole.
Also, public figures are usually more diplomatic in front of the cameras*. They say things like, "my good friend" and "my esteemed colleague" when in reality they wouldn't piss on them if they were on fire. Plus, by all accounts, Trump is an excellent personnel manager, so he's not going to publicly bust on someone he may still have to work with, even if for just a little while.
Comey got busted being dirty and playing politics. He deserved to be fired. The end. After that it's just so much blather and hot air.
*And yes, Trump is more diplomatic in front of the cameras as well. Diplomatic for Trump, that is.