What's clear is that there is no standard of how ann employer treats an employee's time off for illness. In construction and other short-term, project based work the no-show, no-pay principle ususally applies. Its simple logic's hard to argue, but you get infectious, injury-hobbled people showing up, to get full pay just for surviving the day. Management has to either 'police' the sick back home payless, wink at the practice, or play nurse and send them home w/ pay.
Long term, 'permanent employment' needs a different system, because it needs to retain employees, who do get sick over the years. No show, no pay has the good ones on Monster.com all the time. Train points out that sickdays can be used by employers to attract and keep better ones, although it keeps the bad ones too. Whether you managed by outside insurance or inside staff, there are quotas and standards and doctor's notes required. More policing.
In either case, and whether there's a union or not, there's gonna be bargaining involved. Ther'll be a slacker wanting pay for taking his kid's hockey team to an away game, and there'll be a hard worker truly sick up against some rule that cuts her off. And on contract/hiring there'll be someone looking for the 'better' sickdays they think they can get instead of the richer paydays they think they can't. So we add jaw-jaw to policing.
This stuff's about getting along together to do productive work. That's about negociating, whether it's the union contract, looking the other way while he takes a half-day to get braces for his kid, or figuring out how the company deals with the twenty-year guy's cancer. Sometimes, it's about changing what you thought was a good idea, but now see the failings of. Opinions will differ, sometimes bitterly. Just like war's 'diplomacy by other means' a strike is just the negociating process at it's bitterest end.
It's never ever gonna be about one right way and a simple rule. Always talk, and always policing will be required, and there'll always be someone looking to 'improve' their situation to keep the process going.