On the other hand, thousands of (small) US-based companies use part of their home as a business office (and it's perfectly legal).
Correct. The same is equally true of all of the essay-company writers at every essay company I've ever worked for, including the largest and most successful legitimate American essay companies in existence: we're all just freelancers working from our homes. I know of no essay company that actually employs any on-site writers. They all just provide an online portal that functions exactly like a bulletin board: the available company orders all get listed on a secure website to which all of us writers have access, and we just take the orders by clicking the right tabs on a first-come/first-served basis.
The companies never actually meet any of their writers in person (even those of us who have been working for them for more than a decade) and the only way any of us writers knows any other writers is through forums like this one where some of us happen to use the same ID as our essay-company writer ID, because the companies don't provide any of our emails to one another, even when we ask. At most, they'll sometimes relay a message the same way they relay messages between us and customers without ever divulging our identity or contact information. I imagine they probably just don't want us sharing any information about our respective pay rates or about our experiences with their customer-service teams.
Generally, we freelancers are all the exact same writers who are also simultaneously employed by essay companies and whether we get assignments through the companies or independently, we're
all just working from our own homes. There's nothing necessarily "wrong" with that but customers who imagine that essay companies employ fulltime on-site writers all working at the offices of those essay companies are picturing something that simply does not exist, at least to my knowledge and to the knowledge of every other legitimate freelance writer also working for essay companies simultaneously.