I was unaware it was illegal there: I haven't seen anything but machine translation of the original article, but TechDirt provides the detail that the normally open and sane Finns criminalized open Wi-Fi networks after a theft of money that had barely anything to do with open Wi-Fi and everything to do with normal insider embezzlement.
The Justice Ministry is working on a change in rules to allow open Wi-Fi networks without penalty, as there's been little enforcement, and it's just an ugly law.
Does that mean public Wi-Fi without a WEP or WPA key has been unavailable in Finland all this time?
Greetings from Finland. We have had open Wi-Fi here. There's some FON activity, as well as a similar setup using OpenSpark (especially abundant in the city of Turku), and the city of Oulu have for years provided a free city-wide Wi-Fi network in their area. Using them has been legally unclear - I guess if the ESSID gives a hint that the network is purposefully left open, then using it is ok, but connecting to those numerous networks called default, zyxel, etc. has not been recommended.
But as you say, the law has been simply stupid, and not really enforced. But in case of something illegal happening online, using another's open Wi-Fi to do that would bring an additional charge. Or would have, thanks to the upcoming law change.