I think the outcry here is that government has much bigger fish to fry than hounding taxpayers over a few thousands bucks worth of free food every year. Your slippery slope argument isn't too far fetched though: this same sort of tax policy is exactly how the U.S. ended up with its ridiculous employer-paid health insurance scheme.
[1] In some cases. See 91 F.3d 72
I know the article repeatedly talks about employees paying additional tax, but at glance such taxes would seem to be paid by employers?
Also, to go off on a bit of a tangent - I've noticed that the perceived size of the "fish to fry" has a way of changing in line with the predilections of the source of the outcry.
Present the same person with the same dollar value as tax increase/break, spending cut/increase and watch their impression of the relative size and opinion of the change turn on a dime.
It would be paid by the employees. Such expenses are tax deductible by employers regardless of whether it’s a taxable benefit or just a cost of doing business.
Of course the incidence of the tax may be different, for example if, due to market forces, employers have to give people raises to cover the increased tax liability.