job exploitation

What's the deal about employers always expecting employees to do more than what they are contracted to do?
If we had unions, I don't think this would be a widespread problem - but because strikes and riots are criminal, then it will always create this unbalance in the workplace. I am sure that many employees would have been faced with this sometime or other - if it's a one-time favour, then we would be making a mountain out of a molehill, but if it's an expectation, then it's really got to do with employers lacking in correct work ethics.

At the very low rung of the workforce, people are entitled to one day off for 6 days of work - key word, entitled. BUT employers will say, that they cannot get a replacement, so let me pay you extra for not taking an off-day. I don't think it's fair to think that the well-being of our body system could be compensated with money. The money we would need to repair the damage would far surpass the extra the employer is prepared to pay. Also, when you're so low in the food chain, the medical benefits you are entitled to would be pittens, because employers will assume that you're probably not educated enough to know your rights.

Is this a problem in just Asian culture or is this widespread?
Are people just waiting to take advantage of another individual just because he's not as equipped to defend himself from exploitation?

I have done many things outside my scope of work, but I have never gone beyond my contracted hours. To me, my off-time is far more important than money not because I don't need it, but because my sanity and well-being should never be compromised.

I'm helping an old friend tidy up his administration - and he has mentioned getting me to do beyond my 'consult' hours. With a fresh resignation on his table, he needs me to cover some work - he's prepared to compensate me for staying a wee bit longer each day though, but I turned him down, because it's not what I agreed to, initially, extra or not.
I don't have time to work full-time because I need to churn out assignments and a research paper - which to me, is more important than the amount of money he's prepared to fork out. Unless he's prepared to pay for my PhD, there is nothing to discuss - because I need a scholarship, either part or full and the surest way to get it is by delivering good grades. So my opportunity cost is too high for him to absorb therefore this discussion is over even before it began.

My situation is not one of exploitation, because I am setting the parameters, but there are a lot others, who cannot set such parameters and will always be taken advantage of. Although I am able to defend my rights and know how to side-step this request, there are some 50% of the population who do not have the same capacity and will always be in bind to cover someone elses work with no correct compensation ever.