Bukkit/CraftBukkit — comment on commit 76c5230f6329dee94641c396f607041d39d35ee0
File: src/main/java/net/minecraft/server/ContainerEnchantTable.java line 114
By mikeprimm:

It'd cost more to allocate the array needed to do it, (setting something to zero costs just about nothing) and this prevents any possibility that a call made during the event processing that (somehow) would up accessing this class again would see the state.