This is a misunderstanding. If you right-click on the attachment and move it to the trash, you're moving the attachment to the trash — that's it. The parent item will remain in your library and any collections it was in.

When you look in the trash, you'll see the parent item in gray as a context row. The attachment will be in black, because that's what's actually in the trash. This is the same as any search result in Zotero that specifically matches parent or child items — the items matching the search are in black, while context rows are in gray.