I would be very reluctant to do a startup centered on, say, GMail or Amazon Marketplace as a platform. If you get too successful, it's too easy for them to say, "Hey, here's a proven high-value business for us to get into." And then they have my customer right there, so I'm screwed. (An example that comes to mind is Swype. They invented the swipeable virtual keyboard and made a great product. So great that Google added the feature to their own Android keyboard. And then released it for free on iOS, too.)
But I'm not worried at all about using their infrastructure services, because it doesn't give them much in way of competitive advantage. Netflix, for example, has a lot of their stuff running in AWS. Amazon has Prime Movies + TV as a direct competitor, but I don't think Netflix really cares. I as a consumer sure don't; from my perspective Netflix and Amazon are completely separate.