Mmm, this is a messy situation indeed.
It seems to me like a misunderstanding – i.e. they added the permission in consideration of future features, but haven’t yet used the permission to discover that it’s completely unnecessary. I highly doubt the author is abusing this given their status on the market.
That said, I completely understand and agree with your concerns here. You shouldn’t have to sacrifice the integrity of your other purchases in order to activate a theme. Plus, if the author’s database is ever compromised, so too are your other purchases.
You should be able to activate the theme with only a purchase code, rather than signing into Envato or creating an account on the author’s website. This is a strict requirement for all themes that have activation, but some authors are unaware of it.
If you can’t find such an option, I highly encourage you to contact the author and inform them of this so they have a chance to fix the problem. If that doesn’t work, then you may absolutely report the item to Envato Support so they can talk with the author and get this resolved.