As I posted in another thread, my first show was 4/8/85, so I never saw the Dead in the 70s. I wasn't even aware of their music unitl '79 or '80 so I can't answer the question originally posed. But, I went to 3-5 shows per year from '85-'91 or so, and can definitely re-iterate the changes in the parking lot scene during that time frame.
Hershey in '85 is probably the one extreme of my experience. Tickets from the local ticket master at face value plus the service fee. EXTREMELY mellow crowd in the parking lot and in the show. Let's just say that sharing of lots of things was the norm. Police were present, but they didn't hassle anybody who was just quietly drinking, smoking or eating something. I don't remember anyone walking around selling anything, including drugs or memorabilia, which I think had a lot to do with the cops role as just there in case anything dangerous was going on. Folks who wanted some pharmaceutical assistance with their good time could quietly ask around and, if they seemed reasonably cool about it and had a little patience, they always seemed to be gently guided to whatever it was they were looking for. Great show too as I recall.
Contrast to Giant's stadium, either '90 or '91, I don't remember which. Huge hassle to get tickets. Someone went for the bracelet and they ran out, so we got closed out there. About a billion phone calls to ticketmaster eventually yielded enough tickets that all of us who wanted to go got to go to at least one show, but we couldn't all go together because we couldn't get that many tickets to one show. In the parking lot, there were cops everywhere that thought nothing of walking up to where you were hanging out and intrusively observing precisely what you were doing. I saw people arrested for drinking in public (had to be real discreet, but if you had a beer in an opaque plastic cup and you looked of age, they seemed to pass you by), underage drinking, and some guys who were doing whippits a couple of rows over were bundled into a wagon and hauled away. Despite the police presence, there were still people walking around purportedly selling all kinds of things. Not having any knowledge of precisely what is entrapment and what is not, nor how to spot what might be an undercover cop or a dishonest dealer, my whole group decided not to consume anything we did not bring with us. All that for a show were the acoustics really weren't that good.
It wasn't long after that show that my girlfriend and I (now my wife) went to a weeknight show just the two of us. The same hassles were present, and we decided we had had enough of that, at least for a while. A few years later, Jerry was dead. Bummer man.
Mama Mama many worlds I've come since I first left home.