No one owes anyone basic courtesy. However, if one doesn't exercise it, one shouldn't be surprised when he/she gets called names.No one really owes us anything.
It's really very simple. As CL has correctly put it, slush reading is a job. Yes, it's unpaid, and yes, it's your own choice whether to take it or not. Thus:
--If you do take the job and you can't muster enough mental energy to perform it on the weekdays, instead shifting all of the load to Saturdays and then mindlessly grinding through the heap of manuscripts that has amassed itself on your desk--then, pardon my French, you suck at this particular job.
--It is not a bad thing in itself, we all suck at something--but once you realize your own unsuitability, you should inform your employers and step down, freeing the place for someone more motivated and perhaps more capable.
--If you still retain the job, making up excuses for your behaviour, then you'll be rightfully called... something.