In my opinion, there are legitimate reasons for a person (or two) to leave a line and come back. Using the bathroom is the main one that springs to mind. There are a lot of kids at WDW, as well as adults that have incontinence, IBS, or other issues. Couple that with long lines and sometimes you do need to relieve yourself. Also, if a diaper aged kid decides to let go while in line, I would much rather the parents leave the line to change the child in a bathroom and then come back, than change the kid in line.
What I do not think is a legitimate reason is when one person gets on line while the other 5 people in their party shop, rest, eat, or go on an other attraction. Then when they get near the front they call, and the other 5 push through.
I think part of the reason that it is not enforced is it is difficult to differentiate between the "I have been on line for 45 minutes and my kid just loaded his diaper" scenario as compared to the "I didn't feel like waiting in line, so I had one of my family hold a spot for me"