On the one hand I agree, slippery slope is a real thing and exists. On the other hand, I feel this is a slippery slope they could have mitigated the slip on. Say "We will unlock this phone on this occasion only. We understand the risk of not unlocking far outweighs unlocking, and we will hold any key/software in our possession. We may choose to unlock phones in the future, but only if we deem the situation as equal or greater to this situation; one of national security and possible risk to American Lives. We reserve all rights to deny any request for any reason". I see both sides, but I feel this is an instance where they could have done it and made clear enough lines and boundaries where your Average Joe (or small time criminal) wouldn't be impacted and their public image would remain one of "security".

It's a bit ironic that the same American ideals which drove the terrorists to attack managed to keep their information safe, eh?