>It dates back to the constitutional ban on "ex post facto" laws.
Not really, that's not now constitutionality works with respect to the government. Ex post facto is when the government wants to act against you, not when you want the government to behave. They use new decisions regarding constitutionality to undo previous decisions all the time, they just don't want to in this specific case and are using the "well they would have been able to get a warrant anyway if they had known they'd needed one" to justify it.
Not really, that's not now constitutionality works with respect to the government. Ex post facto is when the government wants to act against you, not when you want the government to behave. They use new decisions regarding constitutionality to undo previous decisions all the time, they just don't want to in this specific case and are using the "well they would have been able to get a warrant anyway if they had known they'd needed one" to justify it.