Why is "use strict" still a string literal? [duplicate]
Compatibility across all browsers and JS runtime engines.
E.g., http://ejohn.org/blog/ecmascript-5-strict-mode-json-and-more/
No new syntax is introduced in order to enable strict mode. This is huge. This means that you can turn strict mode on in your scripts – today – and it’ll have, at worst, no side effect in old browsers.