Should companies prevent employees from web-surfing on the job?

Would you want to work for companies that block web-surfing?

Companies are supposed to evaluate workers by their performance, and they are supposed to monitor their behavior.

Others claim that there is no anti-web surfing law and therefor claims that for as long as they get all the work done and everything has the expected performance output, then web-surfing isn't to be considered of a problem in work areas.