I wouldn't draw any conclusions based on Trader Joe's. Their cashiers are friendly and quirky in that same manner around Boston as well. I wonder how they sustain that kind of culture; maybe the pay is better? Some unique interviewing process?
One striking difference between American and German shopping is that in the latter, the staff would rather die than help you shop. I've never been approached by an employee asking "Are you finding everything OK?" for example. Nichts when cashing-out either, you'll be lucky to get a "hallo". I wonder if there's a "don't talk to strangers" principle at work with that? Maybe they simply don't care - not their job - or think you should be able to find stuff on your own if you're not an idiot?
I go to a kayak shop and I can have a genuinely interesting conversation with the guy over designs and why certain ones would suit certain types of paddler, or river. Someone I have never met before, but we share a common passion.
Then again, when I have visited the States, then it seems to be the opposite. You go to any restaurant, the waiter wants to know your name, tell you his, asks if you are having a nice day. It just comes across as false / and fishing for tips (IMHO its crazy that a rich country like the States has such a backward system for compensating employees baked into its culture).
I also feel awkward where anyone is coming across as fake in their customer service interactions, I feel much more comfortable if they just don't do any routine. I would note though that there are places where people have a script but still implement it in a way that shows they genuinely care, which is nice.
Source: I used to work at TJ's.