A couple of things:
First, yes, if you’re out with friends and drinking beer, that’s a social situation where it is expected to share with newcomers (i.e., friends who show up). That’s common courtesy.
I’m talking about perfect strangers. It happened a couple of months ago. I went with a co-worker to buy a soda and there were a couple of young adults standing outside near a bus stop smoking a blunt (marijuana wrapped in tobacco like a cigar). The young man saw that I had noticed what they were smoking and he offered me some. Of course, I said now, but this guy was a complete stranger to me. For some reason, weed makes people friendlier.
Finally, you’re absolutely right about the use/abuse distinction. Unfortunately, sometimes goodfacts (DEA propaganda) are more important than realfacts and people buy into the idea that even looking a marijuana is abuse.