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.

Enforcing the law is one thing. If they refused entry or forced her to fly back immediately, nobody would care much. Detaining is all of: cruel, expensive, unnecessary.

Forcing her to fly back immediately (and detaining until the flight if not immediate) seems reasonable, but both countries at a land crossing can't refuse entry. The article states she was refused entry to Canada, and then detained when she returned to the US; I don't know if there are international norms here, but I think in this situation if both countries would refuse entry, one of them has to accept entry and consider immigration detention; and it doesn't seem unfair for that to be the country where the person in question was before the first crossing?

Sure, they could consider detention. But then there are daily flights back to the UK. Anything beyond an overnight stay (if necessary for the wait) is unfair.

Dictating they buy one of the most expensive flights (one of the immediate ones taking off that day) probably isn't a great look either. Like so much else with law enforcement, they look like shit because of the system and incentives set up.

Some do it themselves and are malicious for no good reason, but not literally every time.

You don't get a free flight. Typically either your return ticket is moved if possible, or the airline will claim the cost from you. There's a number of regulations and airline rules, but in general - unless the airline messed up checks at boarding, you're getting charged for the flight back.