For a small company with a fairly stable feature release schedule, that is fine. But in a large setting where expectations for new features is high, that seems like a scam. That is it looks like they are trying to get the developer to work double (or triple) shifts but only pay for one.
In the (very large) company I work at, the engineers are responsible for writing the code, testing it, and supporting it once it's live. That includes being Oncall and being paged if something goes wrong.