I believe that it is some form of discrimination. Because ultimately you come to work to do just that--work. And if your share doesn't get done then someone else has to do it. However family situations aren't the only case where this happens. I used to work at a restaurant where a majority of the servers were big time smokers. They could take 5--even 10--minute breaks every 1/2 hour without the manager ever saying a word. But the non-smokers? You guessed it...
But what can you do?
