It can be real mistake. Many CPAs just believe that just having customers in USA is not making profit taxable in the USA. Problem is, that law sets rules:
If you are doing activity perpetualy, and if the most of your customers are in USA, company will be taxed in USA. So it is not about only having customers in the USA. If the most of customers are in the USA and you are doing activity for US customers periodicaly, not occasionaly only, company is considered to be a USA︀ tax resident.