https://www.pbs.org/newshour/show/migrants-in-u-s-legally-and-with-no-criminal-history-caught-up-in-trump-crackdown

It irks me a little whenever I hear broadcasters stating that Canadians failed to obtain a US visa.

Canadians actually do not require a visa to enter the US with a few exceptions (entry by marriage engagement for example).

I myself took a bit to understand this process too and this is how I understand it:

A visa is permission to present yourself for entry into the country. You normally obtain this document at a consulate outside of the US. Canadians never do this in most cases.

Once at the port of entry, you then request to obtain status to be admitted into the country. They use the visa, if required, and additional paperwork to confirm eligibility to be admitted.

Whenever you leave the country, you repeat the above process if you want to re-enter. Canadians with work or study status skip to requesting re-admittance and never present a visa.

It's more likely in these stories that Canadians are attempting to renew their work or study status -- I suppose all this is confusing to most audiences so they just say "visa".

The thing is though, it is the border officer's prerogative to ultimately determine admissibility. Having a valid visa (if required) and a previously held status does not guarantee admission and re-entry.

I feel there's more to the Canadian's story linked. Normally if you're given a hard time, you'll be given ample opportunity to withdraw your request to enter and voluntarily turn back. My speculation is that the Canadian in this story insisted to remain in the system (perhaps as a way to contest the officer's decision) and they processed the denied entry while keeping her detained. This denied entry is why an entry ban is now in play; a voluntary withdraw avoids this situation.