Your passwords are stored on their server. You'd have to compile and run your own server, which is more expensive than the $1/month they're asking for.
So you’re paying for the service they offer: a hosted version. You do so because it’s cheaper than hosting your own. There’s no conflict at all with any open source ethic.
> You'd have to compile and run your own server, which is more expensive than the $1/month they're asking for.
For people like me that already rent a VPS for their mail and website the marginal cost is $0 except for the time it would take for me to perform the installation and setup.
If the system is good and stable then the "cost" of the time that I would spend installing it on my server would be close to $0 when divided over the amount of time I use the software in the future.
I think another plus of buying their service is your supporting development of the software and saving yourself time, while a critical piece of your security software remains open source.