Why don’t you need the apostrophe-s? Because English has a category for showing possessives without it … and it’s a rather large category.

• fairy dust
• bear claws
• tiger tracks
• baby poo
• chicken feathers
• lettuce leaves
• castle battlements
• Lady Chapel
• Loch Ness Monster
• angel wings
• devil horns
• cow horns
• deer antlers
• child care
• Jerusalem stone
• Christian ethics
• dandelion fluff
• mustard seed
• computer screen

… O.K., o.k., … maybe it’s only allowable when the person or entity possessing the thing is acting as an adjective describing what type of thing it is.