Why does the C++ standard library always pass std::initializer_list

As a C++ programmer, I have been taught the simple rule of passing parameters:

Passing parameter T by value only when sizeof(T) <= sizeof(void*), except for move_in or output.

However, the C++ standard library seems not comply with the rule. For example, sizeof(std::initializer_list<T>) is greater than sizeof(void*), but std::vector has a constructor:

vector(std::initializer_list<T>, const Allocator&);

Why does the C++ standard library always pass std::initializer_list<T> by value rather than by reference?