Because consumers are also workers, that's how they have money to be consumers. If there are no worker rights, then there is no consumer income, and if there are no workers, because they have all been "made redundant" by a monpolostic oligarchy, there are no consumers.
This doesn't have much to do with workers rights, though.
If a company is in both the grocery industry, and the shipping industry, I see no reason as for how that hurts workers.
There are lots of grocery stores, and lots of shipping companies. Thus, one company does not control all jobs in these industries. It is not even close.
If you want to talk about minimum wage laws, fine go ahead. But that has nothing at all to do with conglomerate companies. It effects all companies, and arguably smaller companies are worse on this front, because they are less efficient, and therefore have less total money to spread around.
