Sync is not only a service. Active (re)configuration can be performed on an ongoing basis, which means it often requires user input.
For example, if you're editing items, changing navigation, etc, you may need to tweak the field mappings, do manual syncs, or reset certain tables to reset them on the webstore. And, to give you some background, historically, much of the web-based configuration that we have now was done entirely through Sync.
Strictly speaking, given how much work is left to be done through the app, it should probably be a service, and we did actually develop a version that consists of two components: a service and a front-end. Believe it or not, there was so little demand for it, that it was not maintained beyond the first version.
I guess the reality is that, if a server is left running 24/7 anyway, it's not so much of a burden to leave the app running with a logged in user. You can, of course, still lock the machine, and that it runs in the foreground makes it easier to manage the config on a day-to-day basis. I'm afraid this is not likely to change any time soon.
If you still are not receiving satisfactory support, please let me know here and I'll follow up directly with the support team.