MacGyver as a television character was very reliant on his hands. He was crafty, and would frequently create things such as bombs while in a dire situation. So the idea that his hands are untied would create the image that his ability is completely released.
On the other hand, "Samson, his hair cut" is a Biblical reference like you said. However, it's got an opposite intention to the MacGyver imagery, in that Samson's power was in his hair. So when his hair was cut he lost all of his strength, as opposed to MacGyver whose strength is unleashed when his hands are untied.