What reason is there for turning a semi-automatic rifle into an automatic rifle ... other than to kill a lot of people?
First, it doesn't turn a semi-auto into full auto, despite what is being bandied about.
The trigger still has to be pulled for each round sent downrange.
What DOES happen, is the device makes the act of pulling the trigger more efficient, using recoil to permit the shooter to fire more efficiently than they would be able to do without it, maximizing their semiautomatic rate of fire. Before these devices, people were doing the same thing with a piece of string or para cord, which requires a little more technique, practice, and skill.
Second, its fun to put a lot of rounds downrange at a (nonliving) target, a test of skill, even though that is a sentiment you may not share.
As regulations go, once again, the question will arise of if some person will be arrested for having a loose bootlace or loop of parachute line in the same pickup as their rifle. All they have to do is say "
any device" and your finger becomes illegal. Put that decision in the hands of the same agency which approved these devices in the first place, and the potential for vague regulation and capricious enforcement becomes surety--all of which amount to infringements on the RKBA, which, if not present would render possession of these arms downright ordinary instead of a notable exception. Were it not for the current legalities, a select fire rifle would cost little more (or even less) than the current semi-autos do.
One of the first egregious BATFE abuses of power I became aware of came with the use of the 1968 Gun Control Act to kick down the door of Mr. Kenyon Ballew
https://topics.revolvy.com/topic/Ken%20Ballew%20raid, and the BATF has been known to abuse its power on numerous occasions, most notably at Mt. Carmel (Waco Texas).
I would submit that these devices are no more dangerous in the hands of those who do no harm to others than a wet noodle. We can't regulate insanity, we can't regulate malice and forethought, for what ever reason.
This is real life, not
Minority Report.
At present it remains entirely possible that one of these could be used for good, too. Eliminate that and only the bad guys will have them.