Yeah, they REALLY need to improve the liability laws around things like this. AEDs are designed to be applicable by untrained users, and tests have shown that people generally are able to use them correctly by following only the verbal prompts.
I checked an in the state where I live you're only protected from liability if you hold a current certificate stating that you're trained in the specific procedure you performed (typically CPR+AED). These certificates often cost $40 and last only a year, so most people aren't going to have them. That is just ridiculous - you should not be liable if you make any good faith effort to save a life.
CPR guidelines generally recognize that even improperly-administered CPR is far preferable to not administering CPR. If the person is unresponsive then CPR should be administered. Modern AHA guidelines instruct non-professionals to not even check for a pulse now - you are only supposed to look for signs of breathing. Even medical professionals are only supposed to check briefly for a pulse before assuming one is not present, since pulses are easy for even professionals to miss. The rationale is that far more people are harmed by a delay in starting CPR than from performing it unnecessarily. Certificates should be even less necessary for an AED - they're designed to diagnose the condition and they will not issue a shock unless an abnormal heart rhythm that is treatable is detected. In theory you can attach one to a healthy person at any time and it won't do anything.