Chipotle is itself a large chain. The incentives to avoid some bad food poisoning the market for one of your 2,000 stores are a little different from the incentives to avoid poisoning your only market.
Looked at another way, the fact that a Chinatown vendor's source would add heterogeneity to a national chain's supply chain doesn't mean the Chinatown guy uses heterogeneous sourcing. He is as localized as his source is.
Looked at another way, the fact that a Chinatown vendor's source would add heterogeneity to a national chain's supply chain doesn't mean the Chinatown guy uses heterogeneous sourcing. He is as localized as his source is.