Because as people have already said, doing a CSS hack so that your <menu> hack doesn't display in the page itself is worse than doing the standards-compliant method of implementing the browser-specific feature via the meta tag.

Here[1] you can see that the standard places no restrictions on what can be entered in the <meta> name option and the browser is not required to implement handlers for all names. Given that this is not display HTML, I think the <meta> tag is the perfect place for this.

[1] http://www.whatwg.org/specs/web-apps/current-work/#attr-meta...