> Nice, just like in the good old days with serfs and feudal lords.
I can't tell if you're being sarcastic or not.
Serfs were largely agricultural workers bound under the feudal system to work on their lord's estate. Foreign workers aren't serfs. They're not bound to a particular estate, nor do they typically execute agricultural work. They have the right to collectively bargain their salary, working conditions, and benefits. They are free to resign their position and leave the country.
The O-1 visa is a temporary worker visa. The expectation is that folks on a temporary work visa work temporarily. That is, there is no intent to immigrate.
You are also not bound to a particular employer. You're only bound to that employer on that visa instance. You can, if you like, apply for another O-1 visa sponsored by a different employer.
There are, in some cases, restrictions on work even for folks that have the right to work and live in the US. For example, if company A contracts out work to company B, it is often not permitted for company A to offer workers at company B full-time positions at company A because of the existence of the contract. If someone freelances for company A they often can't also hold full-time positions at a competitor to company A. Is this feudalism? Of course not.
TL;DR: Freedom to work does not imply or mean, "I can do whatever work I want for whoever I want." There are rules, regulations and laws present for a reason. We can debate whether or not those reasons exist in good faith, but equating "I can only work for one specific employer on a temporary work visa" to serfdom is awfully disingenuous, in my opinion.