That doesn't mean that Americans are more brutally honest in social interactions. In my experience, Americans commonly (admittedly, not always) avoid topics and observations which they think a listening party would find uncomfortable.
This actually reminds me of something I've been thinking about lately. What we call honesty is actually two different things: truthfulness and openness. Americans are probably truthful and not open.
Reminds me of the time I said the fuck word kinda loudly in public in Provo, Utah. Had people looking at me like I'd just grown a second head.
That's kinda why I always say "the fuck word" instead of "the eff word". I've had too many interactions where somebody felt comfortable correcting my word choice for me to be polite about it. ("Fuckin' heck!" is pretty fun too; people just don't know how to respond.)
I am French and we are quite liberal with our swear words. This is interesting because the ones that are used in everyday conversation, due to the intonation, are not rude. Not poetry, but not vulgarity either.
For instance the word "merde" ("shit"). When you say "ET. MERDE." clearly breaking the words apart and emphasizing them - everyone will think "something definitely bad happened" but nobody would be offended. This is only one of the zillion examples.
Now, there is a very, very fine line between being "appropriately, cleverly vulgar" and "vulgar". You do not want to be the latter.
You can say a whole daisy chain and still be fine: "oh putain de merde, quel enfoiré de Word, ma thèse sur Paul Sartre a disparu" ("fucking shitty bastard of Word, my PhD thesis about Paul Sartre has just vanished").
When I go to the US, I always forget to switch on the puritanian-s**t-filter and people look at me really sternly. I even had one older lady coming to my table where I was seated with my teenager boys when she somehow head the word "merde" (as in "et. merde." when I broke something) to tell me that the children should not hear such words.