I'm not so sure this is right - but I'm not a criminal lawyer, so I could very well be wrong about criminal procedure.
From what I've seen, a trial is required to officially revoke a criminal's probation. Given revocation requires a full-blown trial, all the normal rules of evidence, procedure, etc... apply. And because it's a trial, a judicial order is (obviously) required to revoke probation.
From what I've read, the sole issue in Leonard was whether to accept an expert's testimony -- testimony which relied solely on an inadmissible type of evidence. As the Court put it, Leonard was essentially tried-by-polygraph. Given an inadmissible type of evidence was the sole basis for his parole-revocation, the trial court got it wrong and was reversed.
You're right that probation isn't required, and you might be right that a wide array of conditions can be placed on parole (the part about "additional crimes" is wholly irrelevant). What you're getting wrong is the fact that a trial (with all applicable rules of evidence and procedure) is required to "officially" revoke parole. To the extent that a trial is necessary, you can't bootstrap in inadmissible evidence under the guise that 1) it was a condition to parole or 2) an expert relied upon it.