The logic that Disney enforcing fastpass return times would make the standby line shorter and the logic that they should enforce it during busy times are both highly flawed. The rides have the same hourly capacity whether the park is busy or not. If they allocate 50% of the capacity for fastpasses, it is no different than any other day. At Soarin', the same amount of fastpasses are distributed on a Tuesday in September as on Easter or July 4th. There is no reason to differentiate busy from slow in that regard.
Late fastpasses do not make the line longer. If someone's window opens at 10AM and they don't come back, someone from the standby line is temporarily taking their spot on the ride, which lowers the standby line. When they come back at 6, they are taking a spot out of the standby line, BUT if they had come back at ten, it would have pushed the line out then. It makes no difference and is a net equal for the standby line because it's one for one (or two for two and so on). The only time the fastpass line holds up the standby line is when the ride has been down for a significant period of time, so hours of fastpasses are all entering the line at once, which brings standby to a halt because that late person's place in line was not taken, nobody rode during that time at all. Late fastpasses do not otherwise impact the standby line.
What is happening is you are standing in the line and are watching people get on faster than you and forming conclusions about that system that are rooted in frustration, not logic. If you just plan better and get your own fastpass, there is nothing to be frustrated about. It could not be easier or more convenient to use fastpass. Don't get in the standby line of a headline attraction more than two hours after the park opens or an hour before it closes. The frustration is self inflicted on the people complaining and can easily be avoided.