For once I actually agree with Roy. It is a potential injury issue which means workman's comp which does effect the company's bottom line.
Some companies require employees to get help from another worker when lifting an object over a certain weight limit.
I don't know whether the limit should 50 lbs, 70 lbs, or something in between, but they have to draw the line somewhere and calculate to risk/cost/benefit at that limit.
BTW - Backs aren't the only muscles that can be injured. Knees are susceptible too.