You're very right! This is
precisely why they
must avoid a situation where civilians think they have a chance.
Imagine you're one of 20 soldiers guarding a bridge.
4 people come to cross. They look at you, see that you're 20 and don't like those odds, they won't even attempt to cross. But even if they get funny ideas, you can scare them more easily. But even if they're determined, the number of civilians each one of you has to shoot is small: 1 civilian for every 5 soldiers. You can manage. You can direct your shots to be non lethal. You have time to aim and all.
Now imagine it's a thousand people. The civilians look at you, size you up, and they like those odds. They know some of them are going to die overrunning you, but almost no one thinks they will be the one. This makes them go for it.
What's your perspective? You're in a dilemma: you don't want to betray the mission to interdict that bridge, that's for sure. But let's say you're willing to shoot, the deal is the following: if you start shooting, you better not be overrun because once you go that path there's no turning back. This puts you in a corner into thinking "What if they still don't stop" and the number of people each one of you has to shoot on average is huge: 50. 50 civilians for each one of you. In a very limited time frame. You're not in a war zone, most of them are unarmed, and you could know or be related to some of them.
So, do you go on with the mission? Do you shoot everyone? Do you start shooting but then stop shooting? But when you do, you'll be captured, and you'll be captured after killing many civilians..so you failed to interdict the bridge and failed not to kill citizens. It's not like you dropped your weapons before there were any civilian casualties.. Which you can't do either because you have a mission.
This is one of the situations where anything half-assed becomes a blood bath and a "victory" à la Pyrrhus..
Contrast that with a military presence strong enough that sends a signal loud and clear to the civilians they'd better not mess with you because there's no chance they could succeed. Dissuasion to avoid a bloodshed.