Interestingly, there's a pretty universal consensus among tax lawyers that not charging for employee lunches is problematic. Free, regular lunches are another form of compensation and, as such, should be listed on an employee's W-2 as additional taxable income.[1]

In general, anything of value that an employer gives you as compensation is taxable income to you unless an exclusion applies. There is an exclusion in section 119 of the tax code[2] for meals provided by an employer "for the convenience of the employee."

This has was intended to cover things like the cafeteria on an oil rig where alternatives are truly limited.

There was some talk of the IRS publishing regulations that would clarify this, but I don't think that's happened.

[1] e.g. http://taxprof.typepad.com/taxprof_blog/2013/04/wsj-.html

[2] https://www.law.cornell.edu/uscode/text/26/119