What does either of those things have to do with decrypting things found inside either the safe or the iPhone?
If the FBI found coded papers inside a safe, they could try to decrypt those papers, but couldn't compel the owner to assist them.
Security flaws are fair game for law enforcement. Secure encryption without exploitable weaknesses will probably defeat them even in the presence of a court order.
Authourities must first gain access to premises with a warrant and then get a safe cracker in.
VS: authourities (at an airport or just about anywhere) take the phone off the person, there is a layer of obsfucation as to whether a warrant has been obtained or is needed, and data is collected.
If you look at the history of digital data (ie echelon etc) there is a clear trend for vast data hovering of all and any resources. This is not possible with physical data, as it leaves behind a more concrete trail when authorities go after it, and causes them to (generally) be more selective about what they are doing
If you have the same objections when using a safe, the answer to the problem has nothing to do with technology because you believer there is a fundamental flaw in the US criminal justice system. You aren't going to be able to consistently defeat the government by repeatedly trying to outpace them technologically. You have to instead change the laws that govern their actions.
If you think the rules should be different for a safe and a phone, you need to be able to explain why digital evidence should be treated differently than physical evidence?