You should pick it up where ever the dog poos even if the law says you can leave in forest etc...
Any one heard of parvo virus[have'nt a clue how to spell it].
It can kill and live along time in dog poo and if your dog steps in it then go's home and licks its feet it my catch it.If poeple think its ok to leave dog mess in the woods then i hope they never loose a dog to parvo because thats what you could spread if you leave it laying around.