I'd say it's partly due to the setting of the room and largely due to staff laziness;) The room doesn't look very large and seems well-suited for private parties among family and friends, which means birthday parties and babies' first-month parties are big money earners here. And since people don't generally get irked by birthday decorations (much), the staff probably just thinks why bother, especially when the kids' decor can be covered up by curtains.