In deference to Doug, any person can obtain and use a credencial. The rub is when you present it to claim a 'benefit' at the Pilgrim Office. Allow me to explain...
Just by way of explanation and as an FYI aside, we do have a lot of "car pool pilgrims" especially in the summer. These are groups of people, typically youngish, who pile in a car and drive / party from town to town along a Camino route. They obtain sellos / stamps as these are easy to get. Virtually every business in SPain has a rubber stamp.
They, then, fudge the dates to make it appear that they actually walked the required distance. Usually these folks know the rules well, and are seeking to violate them.
Instead of presenting themselves and just asking for a Certificate of Welcome, they request a Compostela. If the staff is suspect, and from lots of experience, they know the signs of an intending cheater, they will engage the 'pilgrim' in conversation, then casually ask if they took photos of the places they visited along the way. If the pilgrim produces a smartphone and says 'si, want to see them?" The staffer will play along, say 'of course' (por supuesto!) and start looking at photos...
Those of you with smartphones know that your camera dates, and location stamps each photo, and can display them in date order with location names displayed. Nearly always, this produces more questions from the staffer about how they could have walked 60 km in one day, yet look like they "just got out of a car..." Alternatively, someone on staff will actually see them get out of a car or taxi around the corner. It happens nearly every day when I am there during the summer as a volunteer. Spotting the cheats becomes a sport to the security staff and volunteers who roam outside the office.
When we can (collectively) make a clear and unequivocal finding of cheating the Compostela is denied and the folks are told to leave...period...
Of course, this could all be prevented if people who do not meet the Compostela issuance requirements just asked for the Certificate of Welcome instead.
Hope this helps.