Because that person’s work is probably scheduled and tracked to a higher degree than most software engineers, and if they’re seen to be slacking off they probably don’t get the excuse of “waiting for the test suite to finish”.

There's down time in virtually every job. If your employer discourages you from using down time to get better at your job, then get a new job. I did it even back in college working at a chain pizza store. I learned some management tasks, how to proof dough properly, etc. I'm sure there's some jobs out there where it's not an option, but I feel like you guys are really stretching it to find ways to say people are incapable of self improvement at work.