Butter beans are called butter beans because of the cream color that butter beans have.
Butter beans have no butter in them unless you add butter to your butter beans.
But the butter beans got the name butter beans because they are the same color as the butter that is made from dairy.
So someone came up with the name butter beans.
Also butter beans and Lima beans are actually related to each other and are the exact same bean and same thing.
Some places call the beans butter beans while other places call the beans Lima beans but they are the same beans.