I don't think traveling workers do anything special with visas - if any visa is needed, they just use a tourist visa. For all business purposes, it looks like they never left their home country; billing is done as if it came from their home business address.
I once spent a few months in Spain, where I devoted 30% of my time to developing a C++ application, 30% of my time to learning Spanish, and 30% on touristy things. This was before the Internet era, and I sent in my work each month on a floppy disk, along with an invoice (just as I did from home). I think they never noticed the Spanish post stamps ;-)