To my knowledge, there's no law prohibiting the scattering of ashes. They're not a biohazard, nor are they toxic to humans or the environment. Some places don't allow it on private property, though. For example, any of the Disney properties. I've heard that people try to secretly scatter ashes in the parks, especially on the Haunted Mansion ride. But security watches guests like a hawk on cameras and if anyone is observed sprinkling ashes, the ride will be shut down and the cremains immediately vacuumed up. Plus, they regularly clean the parks every night/morning, so if you do sprinkle ashes at any of the parks, more than likely they'll be swept away (unless you're sprinkling a tiny vial of ashes in some discreet spot or garden type area/planter). Some places may not allow it due to making a mess in the process. But overall, it's not a big deal if you're discreet and neat about it.
When my uncle died and we scattered his ashes, his friend (a doctor) was there. He told me to reach into the container and take a handful to scatter, that they are perfectly safe to handle with bare hands. We sprinkled some by his favorite tree. By morning, between dew and wind, there was nothing left in that spot.