There's lots of reasons for NDAs besides stealing ideas. I doubt Apple is afraid I'd steal the next design for their new phone (although they may worry Samsung might), but they still probably don't want me blogging about it. There's marketing impact.
Also companies often don't like to commit to product before it's committed. I learned an important lesson at one of my first startups that customers will try to hold you to your "proposed" roadmap. Even though we say, "We're just trying to be transparent. This WILL change. We don't know of this we can do." -- people will tend to find things they like and essentially view what you showed them as a promise to deliver. You suddenly have customers who claim you not only didn't deliver a feature, but that you broke a promise. Until I know I can ship, I really prefer not to talk about it publicly -- even with tons of caveats.
Also companies often don't like to commit to product before it's committed. I learned an important lesson at one of my first startups that customers will try to hold you to your "proposed" roadmap. Even though we say, "We're just trying to be transparent. This WILL change. We don't know of this we can do." -- people will tend to find things they like and essentially view what you showed them as a promise to deliver. You suddenly have customers who claim you not only didn't deliver a feature, but that you broke a promise. Until I know I can ship, I really prefer not to talk about it publicly -- even with tons of caveats.