In spite of laws against such behavior, otherwise law-abiding people can't seem to stop themselves from spreading ashes at amusement parks. Doing so generally means concocting a plan to spread the ashes surreptitiously so as to avoid detection by security guards.
In developing that plan, the bereaved often conduct a lot of online research at amusement park chat forums. On these forums it's clear that not only do a lot of people plot amusement park cremation ceremonies, but they really do carry them out, too.
As they chat with like-minded people, they'll ask whether certain parks allow ash spreading, and when they find out it's impossible to do legally, many pursue the idea, anyway. They'll ask other users how they managed to scatter ashes without getting caught and tips on how best to execute their own mission.
At Disneyland, attractions such as the Haunted Mansion and Pirates of the Caribbean seem to be particularly popular with parkgoers. They hop into the ride car, wait for the right moment and then let loose with the ashes.
Sometimes they get off scot-free, and the only evidence is a pile of ashes that is subsequently swept up and disposed of. Other times, security personnel witness people releasing powdery substances via surveillance cameras.
In those cases, amusement parks obviously take every precaution to ensure the safety of the riders. They'll shut down the attraction, attempt to find the person who did the deed and then thoroughly clean up the mess. In some cases, particularly on water rides, the evidence swirls away, never to be seen again (unless it clogs a water filter).
Every amusement park has security cameras, of course, and if someone's caught dumping ashes there's always a question of what to do with the offender. In many situations, security personnel may admonish the rule breaker but let him or her off without any sort of punishment. Most simply look the other way, opting for compassion over punitive action.
That said, you'll probably never see a news headline about an amusement park pursuing legal action against a grieving family. The negative public reaction would be predictable and not beneficial to the park's bottom line. So the act of jettisoning human remains at such parks will probably continue, on the sly and away from the public eye. So in spite of the rules, grandpa will have his endless rollercoaster rides, zipping around and around into eternity. Or simply disposed of by the park's cleanup crew.