They are banned. Well, maybe not technically banned, but strongly recommended against by the NRMP. Something along the lines of "a program will not try to influence an applicants rank list by sending communications indicating their rank preference" (that's a thumbz quote not an NRMP quote)
Edit: NRMP Quote:
Limit post-interview communication Program directors and other recruitment team members must ensure all information related to the program’s mission, aims and eligibility are clearly communicated to applicants. However, applicants may not have adequate time to obtain the information needed to make informed decisions about ranking and may wish to clarify information following interviews. The recruitment team may exchange clarifying information with applicants following the interview, but must not solicit or require post-interview communication for the purposes of influencing applicants’ ranking preferences. Program directors and all members of the recruitment team should take great care not to promote misleading communication to applicants about ranking intentions and preferences or inappropriately share private information (e.g., letters of recommendation) with outside parties.