Granted, but we also have to hold them accountable for cleaning up their own house and policing their own profession. However, we're talking about how outlandish or "ridiculous" such a claim might be considered - even before the opportunity to present evidence is given.
Just as a counter example, if a used car salesman is accused of fraud, it would still be an accusation in need of evidence to support it. But would it be "ridiculous"? Hardly, since used car salesmen have a longstanding reputation for dishonesty. Or if we say "O.J. Simpson is a murderer," that would also be an unsubstantiated claim, since he was never actually convicted of murder. But it wouldn't be "ridiculous" by any standard.
Let me explain my reasoning, as your analogy here isn't quite the same. Trump claimed it to be "fact" that Obama ordered illegal wiretaps on Trump Tower. He made that accusation publicly, and he did it as his own Attorney General was under the gun for providing false testimony to the Senate under oath. Unless Trump has incredibly bad judgment (which would be bad enough), he must have information that led him to think this was not just likely, but factual. And, he's had plenty of time to provide this information. Today, Spicer said that Trump is not going to provide any information that led him to this conclusion.
Trump could easily ask the FBI, who would certainly know whether this took place. Yet he has refused to do so. And, the head of the FBI came out saying that the accusation was false. So, unless Trump just pulled it out of thin air, he must have information that makes him confident that Obama did, in fact, order the wiretap.
So, back to your analogy, it would be like someone saying "I know for a fact that this used car salesman committed fraud against me in October." Then, when asked for more information, explaining that he can't investigate the salesman without an explanation as to what happened, the person refuses to provide any further information, yet still demands that the cop investigates the used car salesman.