The severity of the foul isn't worth debating. A flagrant 1 foul is defined as "...excessive contact with an opponent.", a flagrant 2 foul is defined as "...contact that is unnecessary, unacceptable, excessive or extreme...". Picking a player up by their leg and slamming them to the ground in such a manner that they hit the floor head first is both excessive and extreme. Case examples often use the term "basketball play" to help the officials determine if the move may have been "unnecessary" or "unacceptable". The only sports where this act would be necessary or acceptable would be MMA and professional wrestling; in Greco-Roman wrestling, the offender would likely lose a point for the head-first aspect.
If you ask me, we have a player that needs to be suspended for their next game, and three officials who need the same treatment due to their either lack of understanding of the rules or failure to enforce them.