Obvious Question Saturdays

Why is it illegal to leave dog poop in grass, but okay to leave a cigarette butt?

I was walking my dog the other day, annoyed that I had to pick up his shit, when I realized a sporadic litter of cigarette butts. Now, I'm hazy on my education, but I do remember something about poop being biodegradable and actually good for lawn growth. 
Cigarette butts are not.
You know what else isn't good for the earth or lawns or whatever you may stand up for; plastic. 
The plastic I have to use to pick up something biodegradable. 
And if I choose to ignore this inane law by leaving my dog's poop on the ground, I could be fined up to $2,500. 

This world is fucked, guys. We have it all backwards. 
Now, have a lovely weekend.