Do people just not know that they're dangerous? Or is it they feel the memorabilia is just that important to them?Yes. Here are a couple examples.
Bomb Team Reponds to Dispose of World War II-Era Grenade Found in Garage
Live WWII German grenade found in Murray
Bomb squad removes WWII grenades, memorabilia from Allen Park home
Unexploded hand grenade found in Sydney WWII veteran's garage - ABC News (Australian Broadcasting Corporation)