Just to help on the idea of monopolies in the grocery sector I offer this.
"Specifically, the top 4 food retailers (CR4) accounted for 31 percent of food sales in 2012 and grew by 3 percentage points to account for 34 percent of total sales in 2019." Source:
A Disaggregated View of Market Concentration in the Food Retail Industry
The biggest nationwide is Walmart (including Sam's Club). One source says they have about a fourth of all grocery sales. Next is Kroger, then likely Costco, Albertsons, Aldi and Target. I say likely because some of the data includes non-food items.
A monopoly is a single seller. An example is your electric provider. Most people, not including their own solar or alternative energy have one choice. Many rural consumers have one choice of grocers in the nearest town, but of course they can shop in other towns or buy online. Interestingly, the grocery business is consolidating over the last 20 years or so. In some larger local markets there are companies that dominate ,but no true monopoly exists nationwide. An oligopoly is several concentrated sellers, one might argue this is the case but if so it would be statewide, not national. For instance, in S. Texas HEB (A great grocer by the way) has a 50% market share. So while some regions might have oligopolies, most of America still has a decent amount of competition.
I am sure they stores will either hire more security, get local police protection or close their stores temporarily if there is a free grab n go. I will say too that churches could really do their part and help those in need of emergency food. Kudos to those already with food pantries or assistance.