Technically he was convicted on a separate charge so he did not go free, but he charges against him for hacking into a WiFi network were dismissed. PC World gives the following explanation:

A computer in The Netherlands is defined as a machine that is used for three things: the storage, processing and transmission of data. A router can therefore not be described as a computer because it is only used to transfer or process data and not for storing bits and bytes. Hacking a device that is no computer by law is not illegal, and can not be prosecuted, the court concluded.

The prosecution had to prove the wireless router was used for storage, processing and transmission of data. That sounds not terribly hard to do (a router is used to store logs and route data, packets are processed and transmitted), but apparently they proved only one or two, not all three. Also, if the law had used the word “or” instead of “and” (storage, processing or transmission of data) the judge might have found a different result. The ruling was appealed.