It's not the same though. If you jump in and start selling merch in Amazon then you are entering _their_ space directly and using their platform to do so. As you said, you're not only handing them money; you're handing them all of your data.
Using AWS to build your back end before finding out that Amazon is now going to enter your market is hardly the same thing. Your handful of VMs make exactly zero difference. AWS or no, Amazon was going to eat your lunch.
That said, you are funding a fierce competitor, even if your AWS bill is just 10 bucks a month...