I think the deal is pretty clear with stuff like this: Free accounts for individual users, earning money with businesses.

It's not like Bitwarden is giving away their product without getting anything in return: The free users (tech-savy early adopters) were the ones that pitched Bitwarden to their bosses when they were looking for a password solution for their company. It's really no different than Adobe or other companies giving away student licences. Companies are not stupid.