The danger is that, thanks to the act of throwing everything into a bin, there is now a practically-guaranteed way to leave a dangerous substance in the middle of a very large crowd and walk far away with no questions asked.
If the person's goal was to bring it on a plane, they're suicidal so they probably don't care if it accidentally blows them up in the security line instead.
If instead they expect to set it down and walk away, they can do that anywhere (even if they clear the checkpoint without having their bottle taken). The difference is that the added "security" has given them a much more effective target than they could have anywhere else.
Before the ban, it was always possible for someone to do either one: get on a plane, or set down an object and walk away. It wasn't happening, which means the old security precautions were just fine. And if it had happened, there probably would have been security camera footage of one suspicious individual setting down an object instead of dozens of suspects throwing objects into a bin.