It is true the machine will deliver instructions and in the U.S. good Samaritans are protected under the law. This does not mean however training in CPR and the use of these machines is not necessary. Every manufacturer recommends training and many states require "medical direction" in their use. The placement of the pads is crucial relative to position and air pockets(which will cause sometimes severe burns) to insure effective delivery of the shock. It is recommended cell phones be turned off or be at least six feet from the unit.
Yes when there is no one trained available a lay person could(and should) operate one but they are designed for use by trained personnel.