Reading the example and rules in the book, it's not very clear. The example never discusses *why* the shield can's be assigned first, and it never explains what's up with the remaining two exclamations (I assume they're discarded).
The reason why all of the symbols have to be assigned is that in later missions there are negative consequences in some of the results. The example does say “The remaining ! is discarded, since there is no room for it on any input” and the second bullet point does tell you to discard symbols that cannot be assigned. The example doesn’t say that the shield can’t be assigned first, and the rules say explicitly (bullet point four) that the symbols can be assigned in the order you want. The reason why the consoles have numbered slots is because you’re expected to get into a situation where you have one or two symbols that don’t finish any of the inputs so you end up making progress but they decided to not make it an amorphous dice pool. Console 2 in Mission 5 (page 27) is one example. Input 1 disables Input 3, and if you have the right string of dice rolls you won’t be able to get that benefit. If you’re stuck rolling a single blue die, you’ll be guessing what sorts of rolls you’ll get in the future. Edit: In other words, delete the words “First and foremost” in the example, and replace it with something like “Because all of the symbols have to be assigned if possible,”. And I think the example just makes a mistake with the ordering of the inputs for Console 1. By majority rule, that “You gain one Software Sample” input should be be Input 3, and the Clamp Release Input 2.
The use of "is" rather than "are" there is very confusing. The example does say that the hit and exclamation have to be assigned first. Or it just means that the order on the card doesn't matter.
It says “First and foremost...” in the example. That’s an ESL error (at worst) that you’re getting hung up on. Or it means you’re just being stubborn. :p The bullet points say you have to resolve them in numeric order, so the order does matter for that.
The error might be caused by whatever, but it's causing a pretty important confusion. Except the inputs aren't numbered.