The FNS was designed as a combat duty sidearm, not a sporting pistol or civilian handgun. Since nearly every military teaches the Hand-Over method of reloading, it was designed with a slide stop that was easy to engage upwards with either gloved or bare hands (since in the engineers' minds, that's all it's there for.)
Last I heard (several years ago), from an instructor who worked with Special Ops troops at Ft. Bragg (Delta, Special Forces, and various Marine Force Recon specialists), the U.S. military changed the SOP from using slingshot or handover to using the slide stop -- after a lot of problems with guns NOT going into battery under combat conditions in Afghanistan and Iraq. And contrary to conventional wisdom, using the slingshot approach still requires "fine motor skills": if you don't release cleanly, things can still go awry.
Some of the problems experienced in the combat zones were due to the fact that many of the troops, of necessity, wore gloves (either because the terrain was terrible and full of sharp material, or because it could also be, at times, be very cold.) The "hand-over" method, while seemingly a solution, can cause problems with the Beretta M9, as a person using that technique can inadvertently decock the weapon when racking the slide.
Perhaps the Department of Defense has changed the standard procedure again, but using the slide stop became the standard practice some years ago. If someone knows of newer standards used by the US military, let us know.
I would argue that whether the lever is called a slide stop or a slide release is irrelevant. That little tab/extension on the lever says that in addition to holding the slide open, the lever can also be used as a release. It seems reasonable to assume that if "handover" or "sling-shot" were the only methods considered or anticipated by the designers, there would be little or no reason for the little thumb-press tab. I think the FN design team wanted their weapon to be flexible enough to accommodate different techniques.