In answer too you're questions (although I may not be correct);
1.) Things such as trees/grass and other natural resources which are either part of someones build or adding anything towards that build for example a tree looking nice next to or near a house are probably best left.
2.) It is true that items time out if left in the "dropped" form, if you wish to leave something for other players you can always place a chest (and probably a sign letting them know its for them) and use /cremove on it to make it public.
Hope that's cleared things up
I will see if i can get in contact with an admin about your tempban.