A UK passport holder does not need a visa in order to enter the US as a visitor. He or she does need a visa in order to enter the US to work, to attend school on a resident basis or to live for a period extending beyond the time allowed to a visitor. The story is that she was turned back at the airport in the UK because her existing visa had been cancelled in response to her very widely publicized testimony that she had used coke on a number of occasions over some years. (Her visa would be dealt with by the US Embassy in London. Lawson was a top story in every newspaper in London and on every news show broadcast in London, for days.) In effect, there were two avenues by which Lawson could be blocked - the visa or, if she had not needed a visa, she might have run up against a US immigration officer who was not willing to let her enter because she is a confessed (past anyway) user of cocaine. In the latter case, she would have been turned back at LAX.
The cases in which a Canadian needs a visa to enter the US are even fewer, and in any event none of the Ford bros. needed one for the recent trip; therefore, in their case the only avenue would have been if they Rob and/or Randy had been turned back at Pearson, or turned back at the airport in Chicago (if they flew to Chicago from the Island rather than from Pearson),* by running up against a US immigration officer who was unwilling to admit Rob due to his confessed use of crack and/or Randy due to his criminal record (although that was long ago and Randy could have applied for and received a US 'waiver' for that long ago).
*They flew to LAX via a stop-over in Chicago.