... if you forget to specify a version number for one of the dependencies, doesn't maven just default to the latest version at build time, thus becoming nondeterministic?
No.
... if you want your build to be deterministic, and of course you do, you still have to specify bar and baz explicitly
If you declare a dependency on non-snapshot foo, then it will also declare a dependency on non-snapshot bar, ergo, reproducible build.
Your comments seem to be the standard "I don't understand Maven, but I hear that it's broken."