An application as sensitive as a web browser should not be proprietary be default. I would argue this is worse than the FreeOffice fiasco, since FreeOffice is not a web-browser, so it does not have to be entrusted with handling so much sensitive data…
I don’t want to speculate since I do not have any access to inside information, but there is, in practice, only one reason why they would ship a proprietary application by default, instead of a fully functioning free software alternative…
I have no problem with Vivaldi being in the Arch repos. Installing it requires my explicit permission.
With Manjaro Cinnamon, an average user has no choice but to install Vivaldi, and then to remove it. It’s an opt out scheme, not opt in. Well, they actually have do a choice – of not installing Manjaro.