They believe civil liberties should be sacrificed because they want to protect themselves from dangers associated with them.
Some people believe that having unlimited freedoms is not necessary and very dangerous to their own and their country's security. Unlimited freedom of speech, press and expression means that many terrorists for example are not being punished for expressing their views, which may be considered a security threat. The same applies to freedom of assembly which can create meeting points of potentially dangerous terror or crime organizations.
The right to protect themselves against unreasonable search and seizure is seen by many as a basic right, it is however, considered unnecessary by others. Searching a suspects's bag may produce evidence against him, and again, proof the fact that some people need to be controlled more.
Civil liberties are questions especially during times of domestic or international tensions, as for example in the aftermath of the 9/11 terror attacks.