According to the standard:

Module code is always strict mode code.

React-Native enables the ES 2015 Modules Babel transformations (see here). However, modules in React Native don't have strict mode code by default.

To reproduce:

react-native init StrictModulesTest
cd StrictModulesTest
cat <<EOT > foo.js
export default function foo() {
delete Object.prototype;
}
EOT

Then insert into index.ios.js or index.android.js:

import foo from './foo';
foo();

and run the app.
Expected: Error Unable to delete property..
Actual: App is showing as normal.

Now insert "use strict"; at the beginning of foo.js and the error shows up.

Note that babel-plugin-transform-es2015-modules-commonjs inserts the "use strict"; automatically, so in some way React-Native must disable this behavior.