Over the past few weeks the Tampa Bay Buccaneers have struggled to assemble a coaching staff. Some coaches have turned them down (like Dan Quinn of the University of Florida), but more often than not the Bucs have been denied the opportunity to interview assistant coaches. Teams can simply block any interviews with assistant coaches who are under contract for any position except head coach. Whether or not you would consider it a promotion isn't relevant: the NFL treats every non-head coaching position identically.
I've heard some speculation as to why this has happened: payback for the Bucs' denying other teams that opportunities for years on end, a result of the late hire of Greg Schiano, a lack of NFL connections among the current front office and coaching staff. More likely, though, this is just what teams do: they don't want to let their good coaches leave. Well, except for the New York Giants, that is.
The Bucs have been denied the opportunity to interview three times now: Ben McAdoo, Green Bay tight ends coach (now quarterbacks coach), John McNulty (Arizona receivers coach, now quarterbacks coach) and San Francisco defensive backs coach have Ed Donatell have all been blocked from interviewing with the Tampa Bay Buccaneers.
But the Bucs aren't the only team to see their opportunities denied this offseason - or any other year, really. According to the Miami Herald, the Dolphins denied Brian Daboll permission to interview elsewhere before they hired Joe Philbin, who let him walk. According to ESPN, the Raiders were denied the opportunity to interview Dallas special teams coordinator Joe DeCamillis. According to the Times-Picayune, the Saints denied the Raiders permission to interview Bill Johnson. Similarly, the Packers denied the Dolphins the opportunity to interview Ben McAdoo.
I could go on for quite a long time listing those denials, but the point should be clear: there is no anti-Buccaneers conspiracy denying them the opportunity to interview assistants. The reason the Bucs have been denied the opportunity to interview those candidates is simple: teams don't want to lose them, and they deny everyone that opportunity.