Before you get on the dyno, you sign the liability waiver anyways. Bottom line, things break and the dyno operator has no way of knowing your trucks true condition and what you've put it through / how close parts are to failing as it is.
When you sign the waiver, you are stating that the vehicle is in good working condition and you understand that it will be pushed and some times things break. It's totally at your own risk.
That's why it costs $100 an hour for unlimited pulls if you're tuning it and it's $150 an hour if you want it tuned for you.
That's also why knowing what the settings and values are is key. Without knowing that, you're in the dark.