For channels to be published as Public channels in roku channel store, we have to pass the certification criteria, There are 6 checklist points given in sdk.
For channels that has SVOD/TVOD, is it mandatory to implement In-channel purchase using roku pay billing?
I believe it is. Roku has to get its pound of flesh. You can try http://partnersuccess.roku.com/ to get a more definitive answer.
If you would like to offer in-channel subscriptions, you are required to implement Roku Pay.
I'd recommend taking a look at our certification requirements and Roku Pay overview docs to get an idea of what is required from developers to build and publish a compliant channel.