This is, I think, the place where the metaphor shear between technical limits and legal limits actually happens.
Legally, we treat reasonableness as the salient divider. To a first approximation, everyone agrees that some searches and seizures are reasonable and some are not, and we've developed a well-defined social technology (warrants, judges, courts, etc.) to distinguish the two cases.
But technically the dividing line is possession of the backdoor. Third parties without a backdoor cannot perform a search, no matter how reasonable it may be, and third parties with a backdoor can perform even the most unreasonable search. There is no mechanical way to introduce a hole for reasonable cases but not unreasonable ones, no matter how well we formalize the decision making process.
Software does kinda work like a form of law, but one without recourse to human judgment. There's no judge, no jury, no prosecutorial discretion. It appears that, without a bit of hands on experience with technology, legally minded folks have a hard time really internalizing how constraining that fact is on what's actually possible.