It is not. I posted on the "ask the ref" forum and much to my chagrin, it is no longer there. The exact rule on unsportsmanlike conduct says,

"It includes, but is not limited to such acts as failure to comply with the direction of the referee, pushing, shoving, swearing, taunting, intimidation, baiting an opponent, throwing ear guards or any other equipment, spitting and the clearing of the nasal passage in other than the proper receptacle, repeatedly droppoing to one knee to break locked hands, indicating displeasure with a call, failure to keep shoulder straps up while on the mat, and failure to comply with the end-of-match procedure."

When you cut someone from his back, it can be considered by the referee to be "taunting, intimidation, or baiting," but nowhere in the rulebook does it explicitly dictate that cutting from the back to the feet is unsportsmanlike.

Long story short: refs have the power to call it, but aren't required to. If I were wearing the stripes I would call it every time I saw it.