Graffiti is by definition uninvited and unwanted (esp. in SF city ordinance)

Then why is Clarion Alley covered in graffiti that hundreds of people a day come to look at? Why is said graffiti often applied by residents?

City ordinance is not an accurate reflection of the desires of all subsections of a city. It's a reflection of the desires of the ruling caste, whose needs sometimes, but frequently don't, align with those of "lower" castes.

A bench is a great place for a nap, unless the mayor happens to see you sleeping on one, gets scared, and calls the cops about it.