I have a question on my engine shops bill on work done.

I just posted about my rebuild 60 series.
It started when my mechanic told me I was missing on at least 1 cylinder and he thought it was a bad injector.
So I went to the local authorized detroit shop
And told him my mechanic told me he thought my missing cylinder was due to a bad injector.
So that was the only discussion, after
Taking the motor apart enough to get at the injectors they found a faulty cam, I said change it.
So they got it back together, new injectors, new cam, and said it still was not working.
Then they tried to figure what was going on and eventually found that the compression was very low in two cylinders. Well I already bought new injectors, new cam and the jake brake was off, so I okayed a rebuild. No estimate was ever offered.
I was billed for 100 hours and paid it.⁰
Mitchell's time labor states that a inframe rebuild for my exact motor with a rebuilt head should be 51.8 hours. I believe that they should have found the low compression prior to putting in any parts.
In otherwoods diagnose the problem first.

Question. Should the engine shop of diagnosed my issue first beforing putting parts in it or as they said, they have to be told to diagnose before they diagnose
Their logic sounds like ########. IS IT?

 

The total hours included taking it apart and puting puting it back together with new injectors and a new cam. Then taking it apart and then doing the diagnosis then the rebuild.
They made assumptions when they changed injectors without diagnosis. And I paid for it.
I think it was wrong for me to have to pay for their assumptions. I would not have done a rebuild if I would have known the compression was bad before parts were installed.