British tourist detained by US authorities for 10 days over visa issue

If she is on a "four-month backpacking trip around North America" and tried to return to the US, she has exceeded the 90-day limit allowed by the Visa Waiver Program (which counts days both in the US and "adjacent territories") and is now an illegal overstayer. The unpaid labor stuff and getting refused entry to Canada is icing on the cake.

For the record, I'm no fan of ICE/CBP, but it looks like they're just enforcing the law here.

There's a right and a wrong way to enforce the law, though.

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?

There is only one way to enforce the law, which is to enforce it upon learning of all the circumstances, both in favour and not in favour of the accused. The fact that you are not familiar with the law never protects you by design ("oh sorry, I never realised I wasn't allowed to just take their bike and ride away") — so it's upon the courts to make judgement on the entire set of circumstances.

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).