And why exactly are the claims of an illegal alien believed? His claims as to his state of mind? Illegal aliens will lie through their teeth because living here is very precious. Which most Americans are oblivious to
Offhand I would say living here as an illegal alien is worth $300,000 over a 20 year period. It is worth more if you are a legal resident (green card) and much more if you are a citizen because then you can import in (legal immigration) your raggedy-arse, ill-educated, illiterate 3rd world family members.
In court, the issue is presence of evidence. Absence of evidence of doubt. The government didn't produce evidence to the contrary, and therefore didn't even probe an element of the crime, let alone prove it. IOW, the alien made the claim, and the government said, "ok."
You asked about the reason, a "why" question. I speculate that the government had been getting away with sentence enhancers for years, using the threat of ID theft, but not taking the time to present evidence and an argument. Laziness, facilitated by compliant courts. A lawyer notice the absence of proof to an element, and lost all the way to SCOTUS.
If you ever get the opportunity to study law, and then see how it is practiced, you'll be amazed at how sloppy and error-ridden the institution is.