It was even more strange for me because often our conversations would be relatively honest and about somewhat personal topics (not just the weather, what cities we'd been to etc) which, in my experience, you only broach when you really want to discuss such things and feel a connection with a person, but, apart from a few people there was still always that feeling that people were just being polite and basically waiting for you to excuse them. YMMV.
In the US, people are casually friendly, but not always polite. In France, politeness manifests as respectful words and actions, consistently. It doesn't hurt that formal politeness is part of the French culture. It's 'hi, how are you?' vs 'good day, sir', because most of the time the 'how are you' is not really a question, but an expression of friendliness.
There is also a fairly clear boundary between acquaintances, friends, and 'BFFs' in France. That boundary is not always clear in the US. It's not as formal as in Germany, where there is a sort of ceremony when two people want to recognize a stronger friendship. In France people (in my social circle, anyway) just switch from formal ('vous') to familiar ('tu'). I've been told that some people have a little discussion about it before switching, but I've never seen that.
You couldn't possible have 20 "przyjaciel"s and spend enough time with them to keep the relationship alive. Same with BFFs. You can easily have 50 "kumpel"s and same with friends.
Keeping this mistranslation in mind I think European (or at least Polish) culture isn't much different from American regarding friendship.
I was mainly referring to the convenience/activity/mentor friends described in the article.
Why should one need a reason to be friendly? Would you rather people be curt and standoffish by default?
Politeness norms. For whatever reason, in the US, there's a certain base line expectation of cheeriness. Especially in service staff like waiters or flight attendants. ("Hi, welcome to McTuckey's. My name's Megan and I'll be taking care of you tonight.")
In my mind, those folks are classified as "acquaintances", not friends. These are folks you might be friendly to, but if you are not willing to, say, take them to the hospital because they are injured, or take them in because they are on the run and someone is after them, are they really friends?