If you’re technically competent enough to have a mobi locally and send it to a kindle, then you’re technically competent enough to convert it, so it’s not a huge deal. I agree it’s weird though.
Honest question: what non-piracy reasons are there for having a mobi file locally and not already having it attached to your Amazon account ready to download straight to your kindle? Did anyone but Amazon ever even sell mobi files?
But you do have them attached to your Amazon account. So there’s likely no real usecase for you to want to push a mobi file to a kindle.