Two "reasons" 0! is one: a) When something is defined by a multiplication formula, the "default value" of the formula is generally one, just like a sum starts with zero (good intro or reinforcement for the concept of additive and multiplicative identity). An "empty product" is generally assumed to have a value of one.
b) The gamma function gives a value of one for 0!. This is a much more valid "reason" than the last one, since the gamma function can define the factorial for things other than positive integers. Of course, the gamma function involves an improper integral, so the situations in which you could use it would be somewhat limited.
(I just joined this list, so I don't know what level people here are teaching).
Now a question of my own--is there a place that I can download an electronic version of the NCTM standards? (Sorry if this is a frequently asked question, but there were no archives to search to determine this).