You can delete and modify non-system apps just fine, FWIW. I imagine Game Center.app is not locked down for some nefarious reason, but just because it falls under "system owned" list.
I think GarageBand also ships with the OS (or at least it used to) so I wouldn't be surprised if it's also on the "system owned" list. This pushes all my security vs. liberty buttons.
I just did a wipe and clean install and it does not install GarageBand.