Edit:
Sorry, should explain. The Lib Dems are the minority junior partner in a coalition government. The idea they would move to kill of their only ever hope at power is a dream at best.
They are also in the second half of the parliament and sooner or later will seek to find issues on which they can distinguish themselves from the Conservatives
It was the previous Labour government (now in opposition) that introduced all the anti-terror acts that have made this possible - this particular one in 2000 - and continually pushed for them to be made more illiberal.
Lib Dems can't and shouldn't move for no confidence in a government which has generally been able to defend liberal policies (admittedly not unanimously), and will continue to do so until 2015.
That action would just serve to endanger the Human Rights Act and the European Convention on Human Rights - exactly the opposite of the stated goal.