YANBU OP.
Had they been adults at the time, I certainly wouldn't have expected my DDs to grieve in the same way as me at the anniversary of their grandad - my father's - death.
If it was the OPs father that had died a year ago, and his mum wanted him by her side for that first anniversary then that's more understandable i think. However this is OPs grandad. He feels he wants to remember his grandad in a different, less graphic, way.
My dear dad passed away 5 years ago. My mum is very into going to his ashes burial site for birthdays and xmas, to lay flowers. She (and he, in fact, when he was alive) always said that when a person passes they are gone, and that attachments to a physical time or place associated with their death was maudlin, and not for them.
She has changed her mind it seems, and that's fine. But at the 2/3 year period after his passing i sensed a bit of upset at the fact that i wasn't turning up at to lay flowers regularly. She has stopped mentioning it lately.
I too like to remember my dad in my own way - as a strong, happy, dependable father. Not the damp patch of ground where we all wept 5 years ago. Grief is a personal thing and an individual should be alowed to express it their own way without being made to feel guilty.