I can justify this, but I'm wondering why Mullvad doesn't give them a slightly better deal. It is basically the same deal that you get if you pay with crypto, except you -- the user -- lose all the benefits of that. So why not charge something like $4.50? Or $4? They are bringing bulk to Mullvad. One of them, or both, could eat the cost until the price stabilized. I'm sure at Mozilla's scale they could push Mullvad's operating price down.
But the fact is that this does create more links in the VPN, and thus more security risks. Which isn't a big deal for the 99% of us that are just using them to torrent and prevent Comcast from seeing our data, but there's still a principle thing, which is part of why people are jumping from PIA before the merger has even happened.