Most countries have copyright laws to protect the creative work of authors and artists. Under copyright law, a fact cannot be copyrighted since it doesn’t meet the requirement of creativity. In many countries that means that the names, dates and places in a genealogical database are not protected by copyright and may be copied without asking permission. But in the Netherlands, in addition to copyright laws we have database laws that may protect family trees.
Many European countries have these database laws. If you put in substantial effort to create a database, then that effort is protected by database law even if it doesn’t meet the requirements of ‘creativity’ needed to be protected by copyright.
Most genealogist put in hundreds if not thousands of hours into creating their family trees. Since we’re not a litigious society, I don’t know think a Dutch genealogist ever sued a person for stealing their tree, but I feel that the genealogical research and expense that goes into creating a tree could meet the criteria of substantial effort. That means Dutch family trees would be protected by database law and may not be reproduced without permission. Indexes and transcriptions may be protected by database law too.
Next time you want to copy a significant part of a Dutch tree, be sure to ask permission first. Even if it is not protected by database law, that would be the ethical thing to do!
Read more about database laws in different countries in the Sui Generis Database Right article on Wikipedia.
[Update: Christian van der Ven pointed out in a comment (see below) that Arnoud Engelfriet, a Dutch lawyer specialized in IT, addressed this topic on his blog. Engelfriet is of the opinion that genealogical databases do not typically meet the criteria for 'substantial investment' so most trees are probably not protected by database law.
As a genealogist who has spent over 10,000 hours and a larger than I want to admit amount of money on travel, photocopies and books to get my tree to where it is today, I would gladly argue that point in court!]