Short answer: He was attempting to show that this was your average run-of-the-mill family.
Up until then, this type of thing was just considered normal. It was really in the mid-to-late 80's that kids started learning in school how damaging these things can be, and I can easily see why someone born later in life would think there's some deeper meaning, but there's really not.
The rest of this is a clarification of that short answer. It goes on a bit, but if you're sitting there thinking "Huh? Average run-of-the-mill family with parents that smoked pot?", read on.
It was 1982, and people in that age group had grown up in the 60's and 70's where smoking pot wasn't such a big deal, and having a drink was just a normal thing to do.
It wasn't until the mid-to-late 80's that we saw a huge increase in in-school initiatives to teach the kiddies young to avoid the evil vices of smoking, drinking, drugs, etc. The combination of the PMRC launching it's war against sex, drugs, and rock-and-roll, along with the rise of groups like SADD and MADD, and later DARE have pushed the idea of teaching good behaviors in schools. These came about later in the 80's.
Just a personal illustration to show how things changed in the 80's:
I graduated in high school in 1987, and even then, we noticed it. My best friend had a younger brother in grade school that came to us wide-eyed one day, saying "There's a kid in our class that's really bad. We asked what he did. Did he beat some kid up? Swear at a teacher? throw rocks at cars? In the same tone of voice that you'd expect a kid to say he shot his own parents, he said, "No! He smoooookes!" (Nodding his head to show he's not making this up.)
We thought it was hilarious, because to us, the idea that smoking was a big deal was just silly, but as time has gone on, we've seen a couple of generations growing up having had this type of education, and as a result, more people find the idea of smoking, drugs, and alcohol, especially on television and in movies to be odd.
Back then, heroes smoked while killing the bad guys. (Think Bruce Willis in Die Hard) Now, it's usually only the bad guys that smoke, or the anti-heroes, largely because of years of pushing from various groups to not show such things in a good light.
Now, probably they wouldn't be shown smoking pot because some group or another would raise a stink. Back then they'd have said "It's an R-Rated movie, what do you expect?"