> IANAL. Why does the existence of the AWA allow such things that the FBI is trying to do?

Short answer: It really doesn't.

But if you can talk fast and convince magistrate judges to issue writs ex parte (no one is there to oppose you), you can get a lot of mileage out of writs that wouldn't survive an appeal. Most people won't have the time, money, or inclination to challenge them.

Even then, magistrate judges have been pushing back. The FBI's going to get slapped for this in the long run. But the long game here is that Comey was hoping public outrage against Apple would convince them to fold and play along with the writ, or nudge congress into giving it the powers it wants. The former clearly isn't working out, but the latter still may.

> The AWA seems to be a very small piece of law, and using it in the way it's being used seems against the spirit of it.

Bingo. It's also against established case law in the opinion of a very large number of people.

I don't think the Congress is really fond of what Comey's doing. The FBI is basically trying to bypass the legislative power by its own means, and to do that it aimed at a target that's too big. The Cupertino hippies might not be well regarded in Washington, but the fact that they are the biggest American company, the most valued and the most respected all over the world still touches some strings in DC.