I used to work a popular attraction in WDW with similar popularity. As others have said, these cast members were doing their job correctly, but there's more to it than that. Cast Members are indeed expected to maintain a ratio of standby to FastPass and it typically runs 1:4. Usually something like 25 standby after 100 FastPasses (we actually had rewards for CMs that could get an exact ratio most often). But that ratio can increase substantially, with CMs getting new ratio instructions from managers based on projected crowd levels. That 1:4 can climb to 1:4, 1:6, 1:8, and the ungodly 1:10. You do NOT want to be merging the line that lets in 10 standby guests after every 100 FastPasses. We did have CMs frequently threatened, spat upon, and in one case assaulted (pushed down).
This had some interesting ramifications on wait times. The average time on a slow day for this particular ride was 40-50 minutes. Never over an hour on its own. If the projected crowd levels nudged over a certain point, the merge ratio was doubled and the wait time would suddenly be 70-80 minutes. Next merge ratio after that, 110-120 minutes. Even if I wasn't working merge point that day, I could always tell the merge ratio by the length of the standby line.
One final tidbit: the assigned merge ratio also impacts the number of Fastpasses distributed. If the merge ratio is doubled, the number of FastPasses is doubled accordingly, and the Standby loading speed is therefore cut in half. The idea is that regardless of crowd level, even on the busiest day of the year, FastPass distribution would last til noon, so guests walking in at noon could still get a FastPasses. Even if means letting in giving away 90% of your line in the form of FastPasses.
Moral of the story: merge ratio affects everything. If Disney ever decides to address the problem with its standby line stops, they should simply permanently lower the merge ratio and give out less FastPasses, running out by noon be d@mned. Let guests get up early. Even if the ratio were 1:2, 10 standby for every 20 FastPasses, standby would still be loading twice as fast as it would be on the average day right now.