They can only sell fruits and vegetables at Boston’s Haymarket and there is a guy who wanders around and fines the occasional vendor who tries to put on something else.  I’ve seen people try to sell batteries, olive oil, cleaning products.  Around the corner from
Haymarket are the remains of the old fish market and one maybe two vendors occasionally show up there; selling only fish.  The stores the line the sidewalk adjacent to Haymarket are mostly meat markets.  They aren’t as tightly regulated and they put on a larger variety of goods.  No doubt you could write a whole book about history of the negotiation between all these vendors and their landlords, e.g. the market owners.

The homogeneity of the vegetable market is an interesting contrast to the shopping mall where the landlord, it seems, strives to maximize the heterogeneous diversity of the tenants.  No doubt the tenants prefer the absence of competition and I assume their leases spell that out explicitly.

I don’t doubt that the vegetable vendors prefer that Haymarket’s regulator stamps out the occasional attempt to change the rules of the game.  The homogeneity creates externalities that the can be observed at larger scale in other geographic concentrations.
Which is why the stifling nature of zoning laws tends to be pretty uncontroversial.  No doubt the folks who live in NYC’s fabric district are peeved when some innovator decides to open a clothing store.

It is a mystery to me why there are so few commercial malls with a homogenous rather than a heterogeneous collection of stores.  Of course there are examples.  Tourist trap malls that sell nothing but trashy collectibles.  Antique furniture malls, that seem take over failed malls.  And of course mall landlords to work to achieve some kind of homogeneity; filling their space with vendors whom will all appeal to the same demographic.  So maybe some of the mystery can be resolved by saying that the homogeneity is on either the buyer side, or the seller side.  Maybe you can’t get high producer homogeneity until you achieve a sufficient population density.