A couple of interesting things.
Note that three lines above the highlighted "Maximum release" it says "Emergency Spillway". It also says "(in saddle on right abutment)" So there was an existing saddle where they built the emergency spillway.
I have a problem with the idea that the "emergency" spillway was designed as an sacrificial plug though. Even if the Main Spillway was inoperable, the emergency spillway is designed to pass a 350,000 cfs flood. Why then would it start failing with only 12,000 cfs flowing over it? Would you really want it to wash out at that low a volume? Or even if its designed to wash out at 100,000 cfs, wouldn't you then say that the maximum release is 100k ? Also, note that the highest recorded flood was 250k cfs. If the Main spillway wasn't working at all, would you want the Emergency Spillway to fail below, say, 300k?
If you want to make sure the dam doesn't overtop there are other, more accurate ways to do it. Along the Mississippi River some of the levies have galleries built into them that can be filled with explosives to remove the levies when they need to release the overall pressure on the levy system. This was done a couple of years ago just south of St. Louis.
Edits made to correct spelling error and to make it more readable.