This is from a morally right stand point and not a rules supported one.
It is supported by the (NCAA) rule. The referee may take any action that he feels equitable. If you feel that treating the foul as a "simple" USC is equitable enough, then go with it.
I personally think that if the runner is stopped close to the goal line, the equitable penalty is to award the score, but this is a judgement call.
The OP did not really specify what "rushing the field" means. If they are simply on the field outside their own team area with the play going to the other side, they probably aren't interfering with the play. But this is a call where when in doubt the non-players did interfere.