The signup bonus doesn't come without restrictions. The casinos are not giving away free money to everyone. Most players probably won't win money with a no deposit bonus, but there are some players who do get lucky and win some.
The amount how much you can cash out from this special casino bonus is usually limited to €50 or €100 (as per T&Cs). The casino will usually require you to make a qualifying deposit before you can cash out the money.
The reason behind this measure is to avoid bonus abuse. For example, players registering duplicate player accounts to get the bonus many times. It also includes playing from a country that isn't eligible to receive a bonus by means of a VPN (Virtual Private Network).
A player who makes a deposit may also have to verify their identity with the casino before a withdrawal is processed. The amount of the deposit has to be turned over at least once. Take a look at the full casino T&Cs to find out more on how to play the no deposit bonus.