A "vacation" is taken with prior approval so the employer can adjust his schedule. Same for so-called "personal days" and other times off not related to illness. A "sick day" is an emergency and expected to be sporadic. The employer doesn't usually have to worry about half his shop not coming in because of illness and the present employees can usually cover for the occasional absentee.
When an entire union decides to run off and use a sick day because they can't get permission for everyone to take off on the same day, they're unconsionably putting the employer's mission at risk. If a school is closed because 75% of the damn unionized teachers are pretending to be sick, all those parents have to call in to THEIR work and explain that they can't come in because their kid's teacher is a mobster. The economic circle of union damage is widened.
No absentee employees who can't show a valid doctor's note should probably be fired or disciplined. Absentee employees proven to be a union rally who called in sick should certainly be fired.
MOST IMPORTANTLY, any "doctor" who issued "sick notes" to healthy union members at a rally should lose their state-issued license to practice medicine and should face criminal fraud charges.