When government gets the ability to spy on its citizens, abuses will take place. The UK well illustrates the problem. Sarah Lyall writes in the New York Times:

... under a law enacted in 2000 to regulate surveillance powers, it is legal for localities to follow residents secretly. Local governments regularly use these surveillance powers - which they "self-authorize," without oversight from judges or law enforcement officers - to investigate malfeasance like illegally dumping industrial waste, loan-sharking and falsely claiming welfare benefits.

But they also use them to investigate reports of noise pollution and people who do not clean up their dogs' waste. Local governments use them to catch people who fail to recycle, people who put their trash out too early, people who sell fireworks without licenses, people whose dogs bark too loudly and people who illegally operate taxicabs.

....

The law in question is known as the Regulation of Investigatory Powers Act, or RIPA, and it also gives 474 local governments and 318 agencies - including the Ambulance Service and the Charity Commission - powers once held by only a handful of law enforcement and security service organizations.

Under the law, the localities and agencies can film people with hidden cameras, trawl through communication traffic data like phone calls and Web site visits and enlist undercover "agents" to pose, for example, as teenagers who want to buy alcohol.