Yes, it's against the law not to (in some states/cities/counties ) and rude as fuck not to. I was a dog walker and cleaned up after each dog that did it's business with no problem. Many of the owners were like "you don't have to! Your job is just to walk them."
If you're going to have a dog, clean up after it. Honestly, I would sue/issue warnings to owners who let their animals poop on my lawn/property. I would actually own a very quiet/vicious dog that would hate other dogs that I don't own just to keep it in my yard...just so people know that if my dog kills yours, I don't care. People are super rude about it but like their cute little dogs...and I don't get that. People would seriously know if I hated them and their dog. I don't care if they don't poop in my lawn (since I have a fence) but if I see them being a lazy dog owner, I will get offended. If someone's dog came and crapped all over their lawn/property they'd get mad too.
You want the animal but you're not going to clean up after it. That's like having a kid and not taking care of it.
I do see people carrying around bags for decoration or because their leash comes with a place to put bags, but they don't clean up after their dogs. It sorta blows my mind but people are just lazy.