I'm not a Swede, but if an random person that I don't know waves at me for no discernible reason, I too would be cautious. Although, it's not clear if you wave at everyone you see, or just at people you need to interact with (such as a store clerk).
As per my example, it is rude to not wave, stranger or not, at least where I grew up. Yet when I moved to the city, entirely different! Same language, country, region. Just urban vs rural.
So it is not surprising to see nation-wide cultural differences.