Being a software developer is different from running a cash register or flipping burgers. Productivity can't be measured strictly by hour and minute. By and large, developers are professional enough to maintain their productivity without companies having to resort to (probably futile) means like restricting internet access. Though I've occasionally been guilty of spending too much work time here or other sites, usually my self-discipline is enough, and definitely more effective than imposed discipline. It's like herding cats, right?