For starters the recoil of an AR-15 isn't as hard on lights as other weapons are.  One of the main reasons for wanting to isolate the lamp from the batteries is to reduce the risk of losing the lamp.  Even though the SF lamps have double springs you still want to prevent the batteries from slamming into the lamp assy. after each blast.  

With horizontal lights you also have slight risk from individual cell batteries banging against each other. Though the risk is small and odds are you will have changed out the batteries before a problem occurs, it is possible that repeated recoil will damage/reduce battery terminal contacts which may lead to activation problems.  To reduce that risk SF recommends(ed) using battery sticks which are 123A cells that are heat shrinked together to keep the batteries from banging into each other.  All of this is done to reduce risk of failures and give the best possible reliability.  

I am not an expert on all the SF reasoning. I still have a lot learn but the above are some of the things SF has already learned. And, it's one of the main reasons why SF is so dominant in this market area & why they continue to add to and improve their weaponslight lineup.