I also think there is a key difference between re-investing in the business and simply bribing customers. I'm sure Amazon sold plenty of products at a loss but they also ruthlessly cut costs and found places where they could truly make money.

I feel like a lot of startups are simply using investor money to give unreasonably inexpensive products to customers.

For software companies, there is often a point where growth simply makes you profitable because your costs are relatively fixed. For my own B2B startup, we had plenty of graphs that showed at exactly the point where we no longer subsidizing our customers businesses and instead making a profit. Sadly we ran into a few road blocks and ran out of cash before hitting that golden mark. But, when your costs rise linearly to the number of customers you have, then you can never achieve profitably through growth alone.