I do not discuss issues like killing someone, bank robbery or making a bomb on my mobile phone. So who cares if a car passing you on the highway COULD hear your conversation. By the way the probability that a bypassing car has the exact same frequency adjusted in his radio AND drives close enough to you is almost 0%. If this almost impossible case happens, he might not even hear your conversation, but what only you collocutor on the other side tells you.

So what?