I think it is mostly the other way round - applications have to be made to disarm them. There are certainly plenty of ex military aircraft around with live seats. This wording is from a googled application although it may well be out of date legislation:
For the CAA to consider accepting that such systems be disarmed, the aircraft must first be shown to have a landing speed low enough (and with benign handling) that it is reasonable to expect the pilot to be able to make a forced landing in a field.....
...... Disablement of ejection seats on individual aircraft may be accepted provided that it remains possible for the pilot to abandon the aircraft, including the deployment of the parachute.
As for rules in the US I have no idea but there are plenty of pictures around of those Tuacnos, with modified canopies, being flown by pilots in T shirts and lightweight headsets, so I presume they are inert seats.
