In what way is what you just described not a tool to crack the encryption?
For starters, they've not been asked to provide a tool. They're being asked to modify the software on that particular phone. Unlocking the phone using the correct PIN will provide access to the data contained in a non-encrypted format. That's not the same as cracking the encryption.
As an analogy, imagine you have a safe containing important documents. Inside the safe is also a bomb that detonates if you try an incorrect combination 10 times, destroying the contents. In this instance what's being asked is for the manufacturer to make a modification to the lock mechanism on that particular safe to allow someone to try unlimited combinations until they can unlock the safe without detonating the bomb. That's very different from asking the manufacturer to make you a tool to break open the safe and defuse the bomb for you.
And if you think it'll work only on one specific phone you're kidding yourself. If you think that the FBI won't make damned sure that they have a copy, legally or not, then you've not been paying attention as far back as when Kevin Mitnick's exploits.
In order to comply, Apple need to build software to work on a particular IMEI and S/N. They've also been told that they can have the phone at their own location to work on it. The software will also have to be cryptographically signed by Apple in order for the phone to accept it. Even if they happened to leave a copy on a USB drive lying around, the FBI can't alter the binary file to work on a different phone without messing up the signature. Heck, even if they left a full copy of the source code lying around, without Apple's crypto keys to sign it, the FBI can't compile and use it.