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The whole point is to get you hooked in the first place.

Scenario

1. You succeed, your startup is now worth something ( big or small ), you will be dealing with scaling issues, people issues, marketing issues, profits issues or gazillion of other small things, the last thing you want is switching your infrastructure provide.

2. You failed, or burn through those cash, but not to worry, since your are backed by incubator or early stage VC, the likely hood of you getting another round of funding is high. And those will be used on AWS anyway.

There are very little downside to AWS but lots of upside.