Clients are sometimes under the false impression that when they obtain a temporary working visa (such as the E-3 or O-1 Visa) they can “freelance”. Freelancing is in most cases completely contrary to US immigration law when it comes to foreign nationals living and working in the USA on a “non-immigrant visa”.  Here’s a summary of what is allowed and what is not allowed:

• L1, H1b, E-3, E-2, E-1 -  May only work “legally” for the US based employer that sponsors them for the visa. One cannot be paid by any US employer other than the sponsoring employer.
• O-1 visa- a foreign national can be sponsored by an employer or an employer who simultaneously acts as an agent for additional outside employers or by an agent for projects with multiple employers.  The O-1 visa thus allows a foreign national to work for multiple employers in cases where they are sponsored by an employer acting as an agent or by an agent.
• If an E3, H1b or O-1 visa holder wishes to work for a 2nd   US based employer they are allowed to apply for and obtain a 2^nd aka “concurrent” h1b, E-3 or O-1 visa and will thus have permission to work for 2 different US employers. This could be in the exact same job classification or even a different job classification if they are so lucky as qualify in more than one category.
• E-2 and E-1 visa holders can be sponsored by a new or existing company that they themselves own.
• Those who obtain an EAD CARD  (Employment Authorization Photo ID card) may work for multiple employers and/or “freelance”.   For the past few years there has been a long delay in obtaining EAD Cards- often 6 to 7 months time from date of filing the EAD application.
• Foreign workers in the USA on H1b, E-3, L-1, E-2, E-1 & O-1 who also have a job in their home country or any other country outside the USA ---that can be done “remotely (“on line”) are free to continue that job located outside the USA whilst working in the USA on their US visa. Note that there may be US Tax implications. Consult an international tax expert.
• Spouses of L1 and E3 visa holders are now allowed to work  for any employer or freelancer upon after entry into the USA (as soon as they  obtain their social security card/number.  E3/S (E3 spouse) or L1/S (L1/spouse) no longer  need to apply for and obtain EAD Cards. This is a huge "gift" as compared to the spouses in any other visa categories including spouses of US Citizens who have to wait 7+ months time to obtain the EAD Card plus pay USCIS filing fees, etc.
• Those who obtain an EAD CARD  (Employment Authorization Photo ID card) may work for multiple employers and/or “freelance”.
• Foreign workers in the USA on non immigrant worker visas such as  H1b, E-3, L-1, E-2, E-1 & O-1 who  already  had  a  job  or  perhaps even own a non-US based  business   ---can work “remotely (“on line”) to continue the  job or ownership duties--remotely  --- whilst working in the USA on their US visa for their US based employer. That being said, for any remote work that is not part of the US Visa sponsored job ----  one must not be servicing  US based  customers  and probably not a good idea to invoice customers from the USA or  be paid/compensated in USA--- especially if one is planning to file for a green card. One cannot  legally do work  for a  US based employer or provide services to US Customers when working remotely for a non US based Company if one does not have a visa to provide such services through the US  employer who sponsored the  non immigrant worker visa.  Before engaging in such entrepreneurial/independent  "remote work"  outside the scope of the US employment based Visa ----  it would be wise to discuss with an immigration attorney. The general rule is one can only work inside the USA for the employer who sponsored the non immigrant worker's visa. The US immigration laws were conceived of long before the internet and remote work was possible so it is not always clear what is and what is not "allowed".