Spectators can open all blocks with guis except for ender chests, command blocks and structure blocks.

Since a recent version (no idea which) spectators can also look into non-container blocks (crafting tables, cartography tables, stonecutters, anvils, enchanting tables), but they still can't look in ender chests, command blocks and structure blocks.

Either spectators should only be able to look into containers, or be able to look into all blocks with guis.