I see an ethical aspect of this in data portability: it should be possible to create your web site in one service and then migrate it to another one for hosting

I see an ethical aspect of this in data portability: it should be possible to create your web site in one service and then migrate it to another one for hosting (it's your content) whenever you like. I wouldn't proxy all requests through though, so at least add a cache, use the original service as a static-site generator, or do an honest import of the data to your service?