Another day older and deeper in debt

St. Peter don'cha call me, 'cause I can't go

I owe my soul to the company store

- Merle Travis, "Sixteen Tons"

The TL;DR of why this is bad is that when employees are paying, or are indebted to, their employers, even in a roundabout way, it's really easy for that relationship to become very abusive.

The company towns also often payed in scrip, which was basically credit at the company store, and, gee, wouldn't you know it, the company owns all the land to the horizon, so there are no other stores in reasonable travel distance! And if you need a little credit because your pay's terrible after your rent (to the company) comes out, and the prices at the store are marked way up, why, they'll be happy to extend some....

That's the extreme version, but it's bad enough that even inching that direction is something worthy of concern. One ought be wary any time money's flowing from an employee to an employer.

The robber-baron version is basically the thing you see in capitalist-dystopia sci fi like The Outer Worlds, but it actually happened.