I live 55°N in Russia so I know that from practice, but we don't lock the windows throughout the summer because the cooling would cost around the same as heating in winter, just one or two opened windows in the night for convection and curtains in the daylight are enough to keep the temperature.
These sentences contradict with each other. The cost is not only payments for service, it's a sum of resources, time and work needed to make and maintain. You can't make a thermal system cool itself passively due to the 2nd law of thermodynamics.
The difference between Helsinki and the place I live is that they don't have temperatures above 30°C while here I have those at least 2 weeks straight each year. Passive ventilation stops in those temperatures because there are no wind outdoors besides the hot columns of air from heated surfaces, HVAC costs are the same as winter's heating while it's PITA to clean them from ice each spring, a window ventilation during the night refreshes the air good enough.
So the "passive" houses are not at all passive in terms of technology (to the contrary, they are quite dependent on technical solutions enforcing airflows and exchanging heat up or down, to the point of being vulnerable; the point is just that this is not done through windows because humans intuitively don't do it so well or optimize for the whole building).
Some years ago, there was an electricity outage in the area where my office was. The basement floor had our server room with plenty of blade and rackmount servers, and some 30 kW of UPS capacity - which had been unused for years because electricity outages are extremely rare here. UPS is all very well to keep servers running, but this happened in the summer and the server room cooling system was not behind the UPS like the servers were. Thus, we suddenly have a 30 kW unbalanced heat load to the basement floor... Result was some frantic running up and down. None of the windows could be opened. We could open garage doors in the basement floor, open all the doors to stairways and halls, and run the the top floor which has the usual sauna compartment with doors to a balcony - opening all these, we could create a chimney effect to get airflow through the premises and prevent our servers from melting on the spot while we were shutting them down gracefully in priority order.