So for a family of 4 you are talking about $6/day, which is $2,190/year. For a working class family with an income of $20,000-$40,000/year it represents roughly 5-10% of their total income for the year to eat healthy.
That's not a small chunk of change for families like that. Sure they'll almost certainly earn that back in lower medical costs in the long run, but in the short term it's a serious burden.