That is not the reason the German backpackers were denied entry to the US.the German backpackers who were detained since they didn't have hotel booking for the entire duration of their five week stay
They had just spent a month in New Zealand. After their month in the US, they planned to spend another month in Japan. When asked how they were able afford such a long period of travel (bearing in mind they are 19 and 18 years old), they said they had earned money from doing freelance work online.
While they did not say they planned to do online work whilst in the US, there was a possibility they could do so, since after all it only requires a laptop. This would be in violation of the Visa Waiver Program under which they sought entry to the US. (Additionally, they mistakenly thought such work would be permitted as long as their clients were not in the US.) Furthermore they had no compelling reason that would require their return to Germany such as school/job/property ownership etc. As a result they were deemed inadmissible.
Normal CBP procedure is to return them to their origin (New Zealand) or their country of citizenship (Germany). They requested to rebook their US>Japan flights and be "deported" to Japan instead, and an exception was made.
The reason they were detained is because of the layout of US airports. When in a departure area of a US airport you may freely walk out, so it was not possible for them to just wait overnight at the gate for their flight to Japan (as would be possible in many Schengen airports or Heathrow). It looks like Honolulu airport doesn't have a 24/7 CBP service where they could have waited under supervision either.
Sadly they were not told that choosing to go to Japan meant they would have to spend the night in a regular jail alongside criminals. If they had chosen to be deported to Germany they may have been immediately put on a flight to the US mainland that would connect to a flight going to Germany, avoiding the dehumanising experience that they had to go through.