It should be the opposite.

Moon sugar should be illegal, while Skooma shouldn't.

I guess it's all because of the actions fromt he Khajiit.

none of it should be illegal since it is so weak it would need at least a lethal dose (not lethal because the skooma/moon suger itself is lethal but because the body cant hold the ammount you would need to ingest and you would expload) to get the smallest buzz, that tree sap on the other hand...that stuff actually makes you slower (supposedly, have not actually checked) and can give you a one hell of a trip (well, maybe it was just me but, the last time i took sleeping tree sap i ran into a spriggin and decied to run...straight into a saber-cat, then i fell off of a cliff and shortly after that i was assaulted by a troll, bad trip man, bad trip)

What about that Purple Tree Sap thing. I remember drinking it and my sight turned purple and I think I was crazy for a second.

http://www.youtube.com/watch?v=sRTUReLBWHA