Despite that Adventure Mode players are not supposed to be able to break or place blocks, they can fairly easily get rid of Powder Snow by setting themselves on fire with any fire or lava blocks. I realize that this is halfway in between a bug and a feature request, but it will be a SERIOUS issue for any map makers that want to create anything with powder snow and fire in close proximity. Since the intent of Adventure Mode is to disallow players from breaking blocks they are not supposed to as part of a set framework or story, this could be considered a bug. A good fix might be a meltPowderSnow gamerule.