I don't think it's a "rule" as much as a "courtesy." And at this point, we need the athletic department to look out for #1 at this point. If that requires not extending formal courtesies, then so be it.
I think it's pretty much a rule before talking to somebody who is under contract with another school. I'm with Junction on this one. I'm sure Bohn has thought about a list for a long time and has talked to candidates who are not currently employed at other institutions, but I'd have some serious concerns if a new HC is hired before there is time for an "official" process.