For my mind, that slide fire mod seems about the most ridiculous concept for a firearm I've seen in some time. <...>
That depends on how you look at it and, more importantly,
where you live.
You turn a semi-automatic rifle into a fully-automatic one - without modifying the action or the trigger. Legally, it's still a semi-automatic rifle as, technically, only one shot is fired by one pull on the trigger. The fact that this method bumps your finger into the trigger at the rate of a fully-automatic rifle makes the distinction largely academic, of course.
If you live in a state that allows fully-automatic weapons, then, yes, this is one of the most ridiculous things out there.
If you live in a state that only allows semi-automatic weapons, this stock is a handy - and legal (at least for the moment) - way of having a semi-automatic rifles firing as fast as a fully-automatic one.
It seems this is how they view it in the US. All other nations' mileage may vary.