Newly setup camera at home, with a 128GB SD card in it for local recording. 

Using the Tapo app on my phone to view recorded detections when away from home.

When I try to download one of these locally stored recordings when away from home, I'm told I need to be on the same network. Why?!?

If it's being bounced through Tp-link servers, then sure there's going to be a bandwidth cost incurred by them (though that's a product design & infrastructure decision they chose, and not strictly necessary).

However you're letting me view the video on my phone, so you've already incurred the relaying cost; the data is already on my phone!

Allowing me to "download" it (save on my phone so it doesn't risk being overwritten on the camera's local storage) should have no cost to TP-link, it's purely a useful convenience for the customer.

Is this some silly limitation being enforced by Tp-link to encourage people to use its subscription based cloud storage?

If so, you've failed. It's trivial to setup a local VPN to make my phone appear as though it's on the same network as the camera.

All you've achieved is annoying the customer with an unnecessary limitation making your product strictly worse than it could be.