well, sitting there when you are there for an exam is a lot different than having to have radiographs taken. the dog needs to be perfectly still and hold a position, and many clinics do not have sandbags that can restrain a dog, and its not always safe to have someone restrain the dog (As they get exposed to the xrays).
in terms of what your dog had radiographs - i am assuming there is somethign wrong with his knee if he needs radiographs? in that case - his knee is probably painful and he may resist having it positioned the way it is necessary to do the radiographs - in addition, some of the views needed are NOT natural positions for a dog, and many require exact angles (90 degrees in each joint) to accurately evaluate the knee.
did your dog blow his cruciate?