If you're building on AWS properly, your usage scales with demand. So a huge bill should mean you have huge demand.
Only servers used for client requests. Dev and integration environments but esp data crunching can be a lot more expensive than serving web requests. And they're hard to keep cheap if you scaling is easy.
Depending on the startup, that may make sense if it allows for fast iteration.
If a startup is trying to achieve product market fit, having a huge AWS bill is a good problem to have, since it means the product is actually successful.