Re: "necessary or appropriate"
Because "All possible assistance" could include requiring Apple to send a copy of your key to Langley. After all you don't want to be aiding terrorism/drug smuggling/child porn/stepping on the cracks in the pavement - do you?
And of course once fitted the feature will be available to other partners, like the police, IRS, INS etc. then other governments are going to want it. Hey Apple, want to sell in China/Russia/South America - well we want access to the same keys.