I'm not sure, to be honest, but I think it's certainly a reasonable position to take.
> IANAL, but as far as I know, if the police can't physically break into your safe, there is nothing saying that they have any legal recourse to compel you to open it.
If it can be established that the safe is yours and that you possess the key or know the combination, I believe a court can indeed order you to open it or to produce the contents, punishable by contempt of court.
> Any why should it be the responsibility of the manufacturer/service provider to supply law enforcement with a key?
Because the state has a compelling public interest in ensuring that law enforcement can successfully execute lawful search warrants. The existence of indestructible safes would constitute a significant impediment to achieving that goal, so manufacturers of such safes have the responsibility of ensuring that law enforcement can access them.
I don't necessarily agree with that argument, but I don't think it's unreasonable.