Exactly: that protection isn't happening right now because those resources are doing something else. The money would be spent anyway, but doing something that is normally considered useful, and that useful thing is not happening to the same capacity as before. Therefore there is an opportunity cost to consider.

The Houthis have been doing a lot of shipping lane disruption, recently. They have sunk several ships.

Iran's Islamic regime has provided material and monetary support to the Houthis.

Crippling their capabilities aligns with the goal of protecting global shipping.