Why is bending moment of an object also considered a measure of resistance to bending?

Answer 1

Because every action has an equal, opposite reaction. So if you bend something, that act of you forcing the material to bend, makes the material attempt to resist the action, hence the term. Plasticine gives much less resistance to the bending action than say, lead.

That's my take on it anyway.

Answer 2

According to the Third Newton's Law .