Is eavesdropping a no-no?

I have pretty sharp hearing and attention deficit disorder. I can’t just block out what I hear. Sometimes I get a kick out of picking up what I don’t seem to be able to avoid hearing. Is this rude? An example: This morning, leaving Safeway, I heard a woman saying to her husband, “Well, we’d better go to the bank. I’ve been dropping twenties like flies.”
People on cell phones seem to speak more loudly than normal. Usually it’s just mundane comments, “Okay, I’m in the freezer aisle now. . . ” Occasionally it’s rather gossipy and I feel I shouldn’t listen, but I do.