This disclaimer has a cultural context to those who did not live in the United States in the '80s (or perhaps the '70s), when a ad (or series of ads?) ran where Peter Bergman, who played Dr. Cliff Warner on All My Children appeared and said approximately the following:
"I am not a doctor, but I play one on TV"
He then proceeded to foist some medical product upon the viewer.
This ad series was dismaying in that the makers must have reeeeally thought the viewer was gullible to assume that someone who played a doctor was a believable authority on medical products. That really says something about the (assumed?) level of stupidity of TV audiences in the US.
Many thanks to generic-man, who filled me in on the details of the original quote, making this writeup much less... generic.