From what I've heard, it's a combination of valuing directness and authenticity, and ironically politeness. Which means one, being very straightforward doesn't feel rude to them like it may in other cultures; two, inauthentic behavior (e.g. excessive smiling or a "fake" personality) is off-putting to them; and three, if you make a faux pas like not beginning an interaction with a greeting, you're perceived as the rude one and they're just reacting in kind. Again, from what I've heard, this is the worst in touristy places where French natives are surrounded by people who don't observe French customs.