It's not a legend but it's now about as common as an honest politician.
At one time, all of the parks had many park specific items that could only be bought at that particular park and nowhere else at Disney. People would often see something at a park, hold off purchasing it thinking they could pick it up later on somewhere else (maybe using a disount at DTD), and then realize later that they never saw it anywhere but at that park.
So all they wanted to do was go in and buy that item but didn't want to use a park admission day off their tickets. Disney came up with a shopping pass.
This shopping pass was available only at the Guest Relations window at the four major parks. The deal was you had to put a charge card imprint equal to the amount of a one day park pass at full price. You would have one hour to enter the park and do your shopping or whatever.
If you did not make it back to the same Guest Relations window within an hour, they put your charge through as a ticket purchase and you became the owner of a one day pass. If you got back in time, the charge slip is voided and it would have cost you nothing.
But now the proliferation of park specific merchandise is very small and everything is available through Disney's phone/mail order department. Guest Relations has all but discontinued these shopping passes. It's possible that you can still find one or two GR CM's who will do it but they are getting fewer every day.
By the way, only one person was allowed to do a shopping pass. You cannot take your whole family in.