I think they just want to use the broadest possible term to maximize sales. Personally, I always think of "customizing" as stuff done specifically to the weapon alone, where "accessorizing" might include holsters, mag storage pouches, gun cases, etc. Some folks might not want to mess with the gun's innards, but could be convinced to buy the other stuff.
Originally Posted by Gunners_Mate
"Placement is power" -- seen in an article by Stephen A. Camp
(RIP, Mr. Camp; you will be remembered, and missed)