Hate to say this but reading the sign that the employees put out, it seems to me that it's a case of workplace violence. In this case, the perpetrator of the violence is allegedly the owner.
Workplace violence and harassment is outlawed in Ontario by the Occupational Health and Safety Act. People in some occupations, such as clerks in smaller retailers, are more vulnerable and at greater risk of being involved in incidents of workplace violence and harassment.
On a specific point mentioned in the poster put up by the employees on the store window, the employees are alleging that the employer or the owner stated "Cancer is no excuse". This alleged statement seems to be connected to another workplace issue: the right of workers to have sick time, if they are eligible. (All of us are eligible for 10 days of personal emergency leave each year in Ontario under the Employment Standards Act. Our employer must, however, has to have at least 50 full time employees; all part-timers and casual are counted as "one (1) employee".
It is doubtful that any of the employees in this retail store qualify for the 10 days of personal emergency leave, as they're probably just part-timers or casual workers.
In the case, as in this one, the employer has allegedly told a worker he can't have a day off WITHOUT PAY because of an employee saying he/she has cancer, the employer simply has the right to ask the employer for a doctor's note saying he's suffering from cancer.
But that is no excuse for being "abusive" to an employee.
Fuji is right. Confident, good employees will not tolerate abuse of any kind and should walk out knowing they can find something else.
And just in case some of you readers out there think I'm just talking from my "academic" armchair about Ontario laws covering occupational health and safety and employment standards, I have personal experience, in a white- collar environment and in a middle-level professional position , of being "abused" by my manager with inappropriate abusive language.