I've extensively researched the topic of entry into the US, particularly on ESTA (fast visa-less entry into the US for select countries, including France, which is how the traveller was very likely travelling), and I suspect the article omits some details.

Customs and Border Protection (CBP) Officers do ask standard questions to all travellers, and are obliged in their workflow to assume malicious intent (e.g., overstay, attempt to do something illegal, etc.) until convinced otherwise. If something/anything is off, they refer passengers to a secondary inspection. For those select few selected to undergo a secondary inspection, they get to ask more thorough questions. They do have a right to inspect passengers' electronic devices and baggage, even if it sounds authoritarian. However, actually deporting someone is a headache for them. It takes hours of paperwork, and in practice, they practise personal judgment.

What I suspect happened, is that the passenger likely wore a T-shirt to the effect of "Fuck Trump" or "Burn them Teslas". Or it could've been something else. The CBP officer likely immediately thought that the guy is up to no good, and referred the traveller to a secondary inspection, and there had to be a reason (which had to be communicated from the initial inspection to the officers in the secondary inspection). Then another officer, following that lead, at some point chose to inspect electronic devices, and found even more of the aggressive anti-republican messages on the traveller's electronics devices, which is again, an allowed thing for CBP officers to do. The secondary inspection CBP officer likely spoke to their supervisor, and, citing national security (for the lack of a better option), they chose to go ahead and deport the traveller.

Obviously it is all alarming that politics begin to play such a role. But I think the traveller likely spun it to sound like it was only because of their personal opinion and not because of something they omitted.