Entering the United States is now affiliated with risks for travelers that may not even be aware of issues that can now result in grave consequences. While prior administrations gave the border enforcement personnel a mandate to primarily welcome lawful visitors with only superficial scrutiny, the advent of the Trump administration is applying much diligence that includes investigating the social media presence of visitors as well as messages and content of mobile devices carried by visitors.
And while the worst case scenario under past administrations was simply the risk of being put on the next flight back to the country the visitor travelled from, an increasing number of reports of cases of detention of visitors that are put away in jail-like "detention facilities" for periods of ten or more days before being able to return to their countries.
Unfortunately, there are many misperceptions of travelers that quickly lead to detention that many are unaware of. Very often such detentions are results of activities during prior visits that the visitors perceive as harmless, with severe consequences.
If you have spent time in the United States after being admitted under a visitor visa or a visa waiver program, you are severely restricted in what you can do while being on U.S. soil.
WORK WITHOUT WORK AUTHORIZATION
In the modern day work environments, professionals such as web designers, digital content creators, consultants etc. have the luxury of doing work from anywhere -- with just their laptop or mobile phone. Often those "digital nomads" are not aware that performing work in the U.S. is not allowed unless you have a lawful work permit. Contrary to common belief, it does not matter whether the work is for clients outside the United States, or if payment is received outside the United States, if the nature of the work is activity that generally is done for monetary value, it is not a permitted activity.
If Customs and Border Patrol has reason to believe that there may be violations on prior visits, it is easy for them to match up dates of visits with projects in which the individual was involved. At unbeknownst to many, it does not even matter if the work was actually paid or not. In the example of a tattoo artist that created a tattoo while on a trip through the United States, Customs and Border Patrol agents found that the visitor unlawfully performed work, regardless if the individual actually was paid for that. What matters is that (like in this case) tattoo art is usually performed as a profession for compensation, and whether actual compensation was paid or not is not relevant.
If you are planning to enter the United States and have engaged in professional activities on visits before, be aware that nowadays, social media provide an extensive record of that. The border agents do have access to these sources, and aside from social media there are also records if freelancers use freelance platforms like Upwork or Fivver that allow a quick look up of the history of paid activities, and if those coincide with a prior visit, there is grounds for removal.
YOUR RIGHTS DURING THE INSPECTION PROCESS
As a visitor to the United States, you have very limited rights. If you are being questioned, you are not allowed to consult any attorney, nor will you be allowed to communicate with anybody if you are taken into custody. You do have the right to refuse access to your phone or to decline to provide your passwords or device PINs to the border agents, but if you do so you will almost certainly be refused entry.
If you do intend to pursue some kind of professional activity in the United States, then it is highly recommended that you look into specific visa, and even if you are eligible for the visa waiver program, apply for a proper visa to the United States. You will be still at risk for any conduct on earlier visits, but the inspection process and risks of a secondary screening (which is the euphemism for being taken to a separate room and being questioned) is significantly lower.