The company is providing a service of vault hosting around the free software they maintain. Hosting and maintenance has many costs that need to be covered somehow, and we want them to improve the service. The hosting costs of individual users with a free though generous account is supplemented by paying companies and governments. Users are not locked in, as they can easily download their vaults and move to another solution or self host it themselves. Some free password managers such as KeypassXC require users to properly manage backups or host their vault somehow. This is too complex for most users and where Bitwarden fits well. Of course the company could go astray, but so far so good.
Password vaults are trivially small to host. The marginal costs for these individual users is small. The bigger concern is if they try to monetize these users in some way that harms them. If they ever pulled something like that, many of us would quickly switch from being proponents to vociferous enemies of the company.
