In Finland receiving as such is not illegal, but forwarding the information is.
In both cases the reasoning is that this information is not directed to the public. Whether anybody has ever been prosecuted I have no idea.
Moreover - when you finally get caught, on which scale do you have to be doing it to get caught in the first place? Remember - laws don't exist outside reality.
And let's be honest - the recording of the presence of a car is also quite illegal in certain jurisdictions. e.g. Germany.
> It looks like it was legalized a while ago
Interesting enough the law has not been changed. The article tells how officials forbade a trader to sell ADS-B receivers to the public. The trader went into court and won. The reasoning of the court was that the law forbids only listening. But because the ADS-B signal is not made audible, but visualized on a PC screen, the law is not applicable.
This was only a medium court. Another one or a higher one might decide differently. Probably you should start lobbying that lawmakers will not "adapt the law to the digital age" when revising it the next time.
In Finnish law the construction is different. All radio communication is considered confidential unless otherwise stated, like e.g. for radio and TV programs. Receiving such confidential information is not criminalized, but sharing it further is.