Does a referring physician in a clinical trial need to be a sub-i?

Because of Confidential Disclosure Agreements (CDA's), sponsors usually provide pre-approved material that can be used with physicians who are not part of the study. In an effort to avoid any CDA violations, it may be easier for Principal Investigators (PI's) to simply pre-screen any potential patients that is referred by other physicians. 

A CDA doesn't have to limit a pi's recruitment options.