The ATF approved bump stocks, because they did not mechanically change the firearm. There wasn't a designated spot for them so they had no ground to not allow them at the time. I've been around firearms for years and have owned one since I was around 19, or so, and so has my spouse (and that's been a long time ago). We'd never heard of them until the Vegas shooting. We asked our dealer about them, how they worked and such, and he said he put one on the shelf, when they came out, but no one was interested , until AFTER the Vegas shooting. Then everyone got curious. Most folks I know wouldn't have one b/c, they're not good for your firearm, is what we've been told, of course, if you're the Vegas shooter, you'er not worried about that.
There are also these trigger mods, that have been approved. This link, explains why we have the Tac Con 3-MR, drop-in, rapid fire trigger, for AR-15s, available to the public. The included video shows the firearm in standard fire mode, and in 3rd mode (which is still not considered automatic).
New firm Tactical Fire Control, Inc., or just Tac-Con for short, has developed a very interesting new trigger that lets people shoot AR rifle very, very fast.
http://www.guns.com/2013/11/20/tac-con-3mr-rapid-fire-non-nfa-trigger-ars-video/
Semi w/a Tac Con 3MR trigger.
https://youtu.be/eQtCTUq4Y_I