I've always thought that the editor knew what was needed to validate a visit and would black out any non-pertinent information if it had to be shared with the client (I would think it is not usually shared). I've never gone in and blacked out information, even when using someone else's CC (such as when I did Mexican restaurant shops with my mom).
The last four digits of a CC number is just enough so that I know what card I used. Of course, then you have Great Clips (not shopped AFAIK) which just prints 16 X's and also XX/XX for the expiration date for "better security." Yeah, and now I have no idea what card I used to pay for my haircut with unless I write it down.
One MSC even wanted me to add in my name and tip amount (using Gimp, Paint, Photoshop, etc) if it's not on the receipt. I would assume they black that out if it needs to be shared with the client.
The way I think about this is: the MSC wants to be able to send the same shopper back in a later month when the rotation expires, so they have "skin in the game" too as far as keeping the shopper's identity a secret. Unless, of course, it's one of those "once per lifetime" shops, and then it doesn't really matter.