As I understand it assult can include the implied threat of using physical force against an individual, such as looming over someone, to compell them to do something. I believe many US states also have related laws about harrassment, meancing, etc.
However, the job of security personel seems to be primarily to discourage activites by being present as deterent against such crimes. Their mere presence is not a threat or assult of course, but if they are attempting to deterre someone from acting in negative manner the security personel 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 personel 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 indentifying someone who's likely to cause an incident and basically pre-emptively start looming nearby them to remind the inebirated fellow that the bouncer is ready to use physical force if necessary in hopes of preventing a situation from occuring. Likewise breaking up a minor fight often involves a good amount of getting between the two fighting and looking meanicing enough that no one want's to try to go around you. Basically you want your bouncer to be able to meance people when a situation is esculating because it's a far safer way of preventing the esculation then allowing it to degreed to violence.
Is a bouncer, or other security individual, guilty of assult or related crime when they do something like this? Does the fact that they are employed by a buisness with the explict 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?