Everything is affected by gravity (pretty much). Blaster bolts are gas combined with light and heat. It's a whole bunch of particles. Heck, light alone is affected by gravity.
The real question is should the dropoff be noticeable by a player in a shooter, and I think the answer is "no". In the movies, we've never seen a blaster/turbolaser bolt go in anything other than a straight line, even on a planet, over long distances (Hoth battle springs to mind). Same with every single other Star Wars game I've played, every blaster bolt went straight until it dissipated.
This is just the developer being forgetful and not disabling/altering the code made for real bullets (engine made for Battlefield and ballistic weapons). If that's not the case, "gameplay mechanics" are the reason, possibly to prevent a situation where everyone is playing as a sniper or something like that, but that's pure speculation.