You can scatter human ashes at a national park. To do it legally, however, you need a special permit. You can also float those remains in a public body of water, but regulations apply there, too. Typically, you need to be a significant distance from shore so that the ashes dissipate before reaching land (and other people), and you have to report your act to the appropriate government organization.
Skirting the rules and scattering ashes in an unapproved way is sometimes called wildcat scattering. For some people, it's an act of liberation and a way to deal with grief. For the people who encounter the remains, it's something else entirely.
It's not that cremated remains are dangerous. Because of the crazy high heat used to burn bodies (often more than 1,500 degrees Fahrenheit, or 815 Celsius), the final ashes are considered to be a sterile substance.
For a lot of people, though, there's a yuck factor associated with encountering human ashes (and sometimes teeth) in random locations. That goes double for folks who hit the amusement park for a day of fun and end up coughing on the windblown ashes of your mother-in-law.
Yet a lot of bereaved people can't help but want to take those ashes out for one last adventure, like an amusement park trip. And when it comes to amusement parks, some are more popular than others.
It goes almost without saying that Disneyland's public stance on ash scattering is flat-out denial. The official company line is that ashes have never been scattered there, it's illegal to do so, and Mickey Mouse and company would really appreciate it if you'd never, ever do it because the idea of dead people ashes and joyful Disney characters mixing on their grounds doesn't really make for the best marketing campaign.
So why would people defy Mickey's orders and insist on scattering ashes at the Disneyland and other amusement parks? Perhaps because those lively, fun places seem like the antithesis of the grim finality of death.