Likewise, when my children were small, I did not encourage strangers to hug or touch them. I believe it a matter of respect. Both children and parrots look to us for safety and protection. Failing to fulfill this expectation results in confusion on the part of our birds, if not a lack of trust. If we are going to ask something of them which runs contrary to their instinctive natures, we can achieve the best possible outcome if we find a way to encourage them to cooperate with our requests while still honoring their autonomous natures.
Parrots operate at all times out of a flock mentality.
In addition, they are incredibly perceptive about our covert reactions to people. Visitors who come occasionally or only once or twice can not possibly be perceived as a flock member. I believe it is an affront for such a person to approach one of my birds and request that it step up. Only those close friends and family members, who come repeatedly, are allowed this privilege. When I have a visitor who wants to handle a bird, I ask that bird to step up on my hand.