Most leases allow entry if the owner feels there’s an emergency situation.
Again an “emergency situation”, ie a persons life is in immediate peril, the police may take action without a warrant of any kind. Further, entry by the landlord because a water heater burst or something of the sort is entirely different than the police entering.
Folks just because we as a society have let no knock warrants go on for decades does not make them legal.
No knock warrants are by definition always unreasonable and thus are expressly unconstitutional.