Cheaper to run than what? If I buy a filter today, that's an added cost on top of my ISP. If I could subscribe to an ISP that does the work for me, then that would be cheaper. Also, many parents aren't as computer-savvy as their kids and aren't sophisticated enough to buy and install a filter that the kids can't get around.

Also, from a legal perspective, why should you get to tell me that buying a filter is good enough. Shouldn't I have the choice of subscribing to an ISP that meets my needs?