From what I've been told, it's hard to charge someone, much less convict him, without more than an admission. There needs to be something to corroborate that a crime/offense even took place. Merely saying you went to place A and paid girl B for sex really doesn't prove the state's case, unless (a) you admit it in court or (b) the prosecution are, at the very minimum, able to place you at the spa.
There was a cold-case murder in San Diego where the killer confessed. The police knew it was a legitimate confession, but they wouldn't charge him because they had no body and nothing to corroborate the confession. Months later they found an eye-witness who said he saw the murder take place. They then charged the killer, and he took a plea-deal and served 8 years. The prosecutor explained that they agreed to the lenient deal, because the killer's original confession and the eyewitness account were not nearly enough to guarantee a conviction.