The buck has to stop somewhere. Employers are responsible for policing the internet they provide in the work place, why not Coffee Shops, Libraries, Universites .... The way the law works you can't be held responsible regardless, the user who finds a way around you filtering is still the ultimate abuser; so long as you take all reasonable steps to mitigate intenet abuse, which is what employers have to do, you are in the clear. I think its a good plan.
Most of our local Library computers are networked to a Central IT Department. They already filter out a load of inappropriate sites, but this does not deter some punters, mainly Eastern European, from attempting to access less wholesome material.
You are right up to a point, there is no direct requirement from an outside body, but there is a very real possibility of prosecution by an outside body. Employers need to protect themselves against the actions of their employees (i.e. from vicarious liability).