Just because things were originally considered safe - or more likely not considered at all - doesn't mean that they continue to be thought safe. People have "whinged" after accidents on the job, whether severe, like deaths in mining, or less so but still significant at times as in office work.
Note also with your analogies that the comparison fails. In this case, they are talking about the extent and severity of the task at hand, rather than the job itself. They're not saying they won't load luggage at all (or luggage of certain characteristics, e.g. vinyl). They're saying to keep it reasonable and not excessive. Perhaps a better analogy would be if your teacher had to write 30 copies of each handout by hand instead of just one - permanently, because the school wasn't going to pay for copies.
Changes have happened to working conditions in different industries reasonably frequently.
I find it odd that you think considering the safety aspects of different jobs and that maybe considering their situation is winding you up.