Well, the USS team is already into the city to eliminate all possible threats to Umbrella. Maybe they were the closest available unit to the laboraory when Leon infiltrated it (e.g Maybe they were at the RPD at that time).
Besides, the game is obviously non-canon. Leon never got attacked by any member of the USS in RE2.
Anyway, like I said i don't expect the storyline to play an important factor to this game, because it changes every time you play and because it's a 4 player co-op game. It's gameplay that keeps those games alive imo.
Thats what i was getting at...was there was a team on the ground trying to kill survivors...Leon is a survivor, and he could expose Umbrella with what he found down there...And granted its non-cannon...but at the same time it does give us a little insight, becuase how much you wanna bet that it gives us all sorts of clues to Umbrellas ACTUAL motives and plans at the time...