The major problem with ISP filtering that people seem to brush over is that it only works while using your computer/internet device at home. As soon as you walk out of your home with your laptop/iPod/tablet etc etc and hookup to a different wireless network you are completely unfiltered. In addition, if you (or any one else in your house) can get the wireless from your neighbors who don’t have ISP filtering (or live in a city like Miami Beach that has free open wifi throughout the city), you can easily get unfiltered internet. In all the above situations if the filter was on your device you would still be filtered. Obviously there are many limitations from having a filter on your device as well (look at above comments).
Ultimately, people have to understand that “just installing a filter” is not only not so simple, it’s very often very easily bypassed one way or another. You might not know how to get around it, but trust me your nine year old does.