First, because priority is for things like major feature work, so that engineers can find the bugs that are useful to work on. In this case, everyone in the team responsible was already spending 100% of their time addressing the issue.
Second, because we care about solving problems, not being bureaucrats.
Really? Because being the bottleneck (i.e. single point of failure) responsible for approving all addons is exactly what bureaucrats would want to do ;-)
The non-bureaucratic thing to do, as has been pointed out many times of course, would be to give users the power to override the cert signing check as an advanced option.
It is, in fact, an "advanced option", in about:config.
But that option only works in nightly builds, not Firefox release builds. If an option doesn't honour what it claims to, imho it might as well not be there.
You have the book because at one time you didn't have the book.
"The book" is something that needs to change and improve like anything else. Sometimes, that means you'll still need to wing it and add that thing to the book later.
However, I bet it’s likely they have procedures and policies for work that first involve signaling like for example the priority level.
I’d be willing to bet lots of things surrounding this issue weren’t handled in a by the book manner. So if you are always going to wing it, why have a book (or a public priority level system) at all?