It's not just GitHub and it's not just because they don't want to pay bug hunters. In my career, I have escalated multiple bugs to my employer(s) in which the response was 'working as intended'. And they wouldn't have to pay me another cent if they acknowledged the issue.

In my experience, there was two reasons for this behavior:

1. They don't want to spin dev cycles on something that isn't directly related to revenue (e.g. security) 2. Developers don't have the same mindset as someone who's whole job is security. So they think something is fine when it's really not.