If I've just downloaded a new app, I'll have it ask for permissions for every disk access it needs. After a few runs I'll start giving it permanent access to the dirs I'm OK with it using. No app gets to write to /Applications.
It's a little painful to deal with the pop-ups but I like to know what my apps are up to :-)
As non-admin, you have authenticate to create or delete items in /Applications/. However, all items you move to /Applications/ remain under the ownership of your user.
You can confirm this without even opening the terminal: move the directory Foo/ to /Applications/Foo and notice that /Applications/Foo/bar is user-writable.
(Furthermore, admin on OS X (and many modern Linuxes) isn't equivalent to the traditional root account. Using a non-admin account doesn't make the difference you think it does.)
