I think they try to do it too fast. They should get into a decent program and let their bodies develop rather than try to bulk up quickly.
But the general mentality in NBA scouts etc is "needs to get stronger." Geez, it says that in nearly every scouting report for every player on NBAdraft.net.
When you add upper body strength, you still add weight and you still have to lift it up off of the floor when you jump. So it is inevitable that you sacrifice some quickness even if if you don't sacrifice vertical inches on your jump shot.