A PI can be just a guy with a car and a camera though. You can't expect them to be able to handle the degree of regulations banks have. Your link says they were hired under false pretenses.
There's more detail on the allegations here: https://www.justice.gov/opa/pr/iranian-intelligence-official...
> As alleged, Bahadorifar provided financial and other services from the United States to Iranian residents and entities, including to Khazein, since approximately 2015. Bahadorifar facilitated access to the U.S. financial system and institutions through the use of card accounts and offered to manage business interests in the United States on Khazein’s behalf. Among other things, Bahadorifar caused a payment to be made to a private investigator for surveillance of Victim-1 on Khazein’s behalf. While Bahadorifar is not charged with participating in the kidnapping conspiracy, she is alleged to have provided financial services that supported the plot and is charged with conspiring to violate sanctions against Iran, commit bank and wire fraud, and commit money laundering. Bahadorifar is also charged with structuring cash deposits totaling more than approximately $445,000.
> Farahani, Khazein, Sadeghi and Noori are each charged with: (1) conspiring to kidnap, which carries a maximum sentence of life in prison; (2) conspiring to violate the International Emergency Economic Powers Act (IEEPA) and sanctions against the government of Iran, which carries a maximum sentence of 20 years in prison; (3) conspiring to commit bank and wire fraud, which carries a maximum sentence of 30 years in prison; and (4) conspiring to launder money, which carries a maximum sentence of 20 years in prison. Bahadorifar is charged with counts two, three and four, and is further charged with structuring, which carries a maximum sentence of 10 years in prison. A federal district court judge will determine any sentence after considering the U.S. Sentencing Guidelines and other statutory factors.