She was literally living for free in the USA in exchange for doing home chores, childcare etc. That clearly requires a different visa to a tourist one so the Canadian authorities did nothing wrong. The article mentions Workaway twice, although glosses over the fact whether she was doing that while this incident happened (strongly suspect yes). If the platform is called Workaway, it's obvious you need a working visa.
She was also the nanny for that Guardian writer's children for two years so it's hardly unbiased journalism. The entire story is tragic but still caused by a bureaucratic blunder. It smacks of if you're white, female, middle class and draw comics then you should be inexplicable exempt from basic laws. (1984 - All animals are equal but some are more equal than others). If a male POC ended up in the exact same position by mistakenly not organising a working visa, the Guardian wouldn't be touching his story with a 10 foot pole.
Even the headline isn't technically correct because it calls her a British TOURIST. She was in the USA working under a tourist visa. It's the classic scenario that happens often with illegal immigrants who outstay their tourist visa and end up working unregistered. So the fact she ended up at a detention centre with POC immigrants who did the same thing on paper is no surprise at all. Why should the fact she's white make it more tragic and Guardian-headline-worthy that she had to spend time in a cell?