Points per game is a really bad measure of defense. At least look at points per possession if you want to get something closer to the truth, so that you're not misinterpreting the faster pace of the game as worse defense.
That said, I'd expect even points per possession to be higher than it used to be, because offenses are so much better. I'm not just talking about the quality of the players, but the superior strategy that's used now. It's as if NFL teams went from throwing 5-10 passes per game to throwing 25-35. Of course, defenses would be allowing more points.
Modern NBA offenses are much harder to defend against. It's nearly impossible to just play good man-to-man defense and shut down a good offense, so not only do I see players going all out on D, but they mix in a healthy dose of switching to limit the impact of top players being able to get free from almost any defender.
It's actually hard to think of a really good NBA team right now that doesn't play pretty good defense. Maybe Dallas?