Al....
I do not speak for the Avast vendor, however, when a product is as good as Avast, then you want to let people know not only about the quality of the product, but the support available.
What better way then to offer a freeware version!
As you have seen, once you get a taste of the Home version, then the Pro is the way to go. It may not be their only motivation, but it does pay the bills.