As I understand it assault can include the implied threat of using physical force against an individual, such as looming over someone, to compel them to do something. I believe many US states also have related laws about harassment, menacing, etc.
However, the job of security personnel seems to be primarily to discourage activities by being present as deterrent against such crimes. Their mere presence is not a threat or assault of course, but if they are attempting to deter someone from acting in negative manner the security personnel would likely have to get close to the person and make demands that they leave or stop whatever activity their engaged in in a manner that could be considered a threat of force if they don't comply.
To go with a particular difficult example think of a bouncer at a bar. More then most security personnel a bouncers job is practically defined by their ability to loom. My father owned a bar, and as he described it to me a good bouncer should be identifying someone who's likely to cause an incident and basically pre-emptively start looming nearby them to remind the inebriated fellow that the bouncer is ready to use physical force if necessary in hopes of preventing a situation from occurring. Likewise breaking up a minor fight often involves a good amount of getting between the two fighting and looking menacing enough that no one want's to try to go around you. Basically you want your bouncer to be able to menace people when a situation is escalating because it's a far safer way of preventing the escalation then allowing it to degreed to violence.
Is a bouncer, or other security individual, guilty of assault or related crime when they do something like this? Does the fact that they are employed by a business with the explicit task of keeping the peace give them any extra leeway with such actions? Is the fact that they are trying to prevent/discourage other crimes give them more leeway?