For the record, I'm no fan of ICE/CBP, but it looks like they're just enforcing the law here.
Putting her in a literal prison and in an orange jumpsuit is overkill. Clearly she just screwed up and thought what she was doing was ok, but isn't a threat. Let her go back to the UK and no longer be eligible for ESTA. How is that not sufficient?
The problem with any immigration service in the world is that they are dealing with non-citizens which lack most protections citizenship would have given them — which means that it may take its sweet time before courts actually hear her defense and probably decide as you suggest (along with introducing a 3 or 5 year ban on entering the US).
Treating every possible infraction like a hard felony is both costly to the taxpayer and chilling to tourists.
The approach of ICE appears to be: Nobody has rights, no search is unreasonable, harvest all the data, then lock everyone up and put them at the back of a hearing queue.
Post-9/11 America is a horrible border experience for legitimate tourists. It is repeatedly noted about how unwelcome you are and how very little you need to do to be incarcerated and deported.
They catch and detain people. Sometimes, some of the people with jobs, homes, families —something to lose— get bail after a hearing, but they only account for ~50% of ICE detainees. When you also factor that the average ICE detention is over 50 days, many of the people who attain bail, are only doing so after many weeks in prison.
I'm not saying you agree with that, but you seemed to be [at least partially] rebutting the previous comment. I think this case, and many like it could be dealt with that don't involve being kidnapped at the border. Either just rebuffing their entry, or trusting them enough and allowing them to return home via an internal airport if they already have a return ticket.
Or even just trusting people until they've committed a crime. Many people with border issues are merely suspected that they might be intending to work, intending to stay.
I am pretty sure the law does not have an option of "be escorted to the nearest international airport/exit at deportee's cost", but in this case, having an option to leave yourself in 48h or face even harsher bans would have likely worked.