On the rare occasions that I have an excuse to wear flip-flops I find it tricky, and thereby very annoying, to drive a car in them, so try to avoid it. Even riding a bike in them's a bit of a bugger, as is running.
If I forget to change into more sensible shoes then I just drive barefoot, which isn't so bad.
From experience, I can see why the lack of 'bond' betwixt foot and flip-flop can lead to reduced control over pedals, causing concern for safe driving practices, but 'illegal'? Never heard of that before.
Also, given my good fortune in being able to visit hotter climates in my lifetime, where many more people wear such footwear and drive daily, surely they manage without legislative intervention as I've never heard of it being outlawed elsewhere.