They're allowed to lie to suspects, not the courts where parallel construction happens. A defense attorney can't get evidence stricken when it came from dubious sources when they lie in court about the source.
> The two senior DEA officials, who spoke on behalf of the agency but only on condition of anonymity, said the process is kept secret to protect sources and investigative methods. "Parallel construction is a law enforcement technique we use every day," one official said. "It's decades old, a bedrock concept."