In the opinion of the referee?

[whistleblowerusa]

Ah, good to hear from you. That has been one of those myths that somehow got started and I don't know where.
On the US Soccer Pro web site theer is a listing re-affirming what we are supposed to do. They have stressed this to referees at the top level but it is also meant to be used at all levels.
Try http://www.ussoccer-data.com
You will need a logon for most of the stuff there but you can access the Topics page. On that page you will find many good things that are new or re-worded. You will also find a topic Misconduct and showing of cards or something like that (I haven't looked at that one lately). It is a PDF document and you will need Adobe Acrobat to read it.