The Chromium project does not distribute binaries, or installers, or uninstallers to end users. You should complain to wherever you got Chromium from. Maybe it was bundled with malware?
Personally, I'm using Google Chrome on several computers at home and at work, and it does not autostart (why would it?), and it can (of course) be uninstalled.
Maybe what you're seeing is the feature that lets apps/extensions run in the background. There's a checkbox "Continue running background apps when Google Chrome is closed" in the advanced settings that turns that off if you don't like it.