Heyskull, you're pretty much right, in my opinion, but I think they are thinking they don't hafta look for the simple things. I think they wanna know what size something will be on a vehicle, or the colors are basically correct, or maybe negative space and balance. I don't think they are checking every word for spelling errors.
Perhaps, if you want them to take responsibility for errors from your side of things, you may want to add something like....... all spelling, grammar, punctuation, colors and whatever else you need them to check for you outlined in much bolder copy on your proof sheet.
We just did one for a company in which we designed everything. They Okayed everything and were happy with it...... they had picked the colors, gave me the wording and I hadda explain why and how I placed everything. They saw placement on the trucks and machinery with proofs. We then lettered several pieces and vehicles. They then asked me to send the file to a screen printer for making jackets and shirts. I did that, too. They came back about a week later and said they thought the second line of copy was a little too small. Why now ?? I asked. Well, we just thought it should be bigger. Hmmm, Okay, so, do you want me to change your logo for all future jobs ?? Yes. Alright, I won't charge for it, either. When can you change the trucks and stuff you already did ?? I must schedule and the cost for removing certain things and replacing will be........$$XX. Why ?? You had already given me a complete thumbs up and paid for everything and were happy before. Now, someone has put a bug in your ear and you want me to change it for free ?? Nope, ain't gonna do. So, they said all future stuff will remain the same, then.