Xfinity charge is based on FAQ that says no charge during beta. But will be later.
Verizon has 1/5 of the customers of xfinity. There is a cost to develop and support an app. More customers makes it more cost effective to support.
Verizon just spent a ton of money on developing new WiFi based stb for new IP based TV service. For some reason it was shelved. Just shows how hard it is to develop and launch a product.