A well made point! I feel that people look at this too lightly. It's not about weather or not they would or would not be in an accident, because yeh, that is minimal, but it's about what you represent and how you present.
And also it is dangerous for a baby to not be strapped in correctly in an infant seat like this, outside of the car.
I think we too easily forget that there are dangers outside the car to take into account.
And yes, it's a "one in a 'million'" chances that Will would have dropped the seat, banged it, tripped, that someone would have come barging out of the crowd spooking him and so forth, but that shouldn't matter. Safety first, because you never know.
Just like the saying goes: "You don't stop wearing a condom just because it's a quickey". ;)