How does employment internationally or on a visa work?

Now, I'm working on getting a writing job through a friend, who has a large stake in a company. He (and the company) are in the US, I am not. Can I legally be employed by that company? My assumption is yes. But I'm planning on studying in (and eventually moving to) the US, and a study visa does not give me permission to work in the US. So would I then have to quit my job, despite doing the exact same thing as before?