Honestly, I think you are misunderstanding the generalization (and it is not a Celtics fan generalization, it is something that is firmly implanted in the national media coverage of the NBA). It is not that the West doesn't play defense, it is that the Western Conference is generally a more up tempo, fast paced, high-scoring game, while the East generally is known for a slower, more physical game. With any generalization, you can pull out specific teams and say it doesn't work, but the longstanding assumption has been that on the whole, there was this difference in styles.
I don't think it is nearly as prevalent than it was in the past though. In the last couple of years, a lot of teams in the East have been picking up the pace, while many of the top Western teams have been slowing it down. At this point, you are just as likely to see East coast teams opening it up, and West coast teams slowing it down.