Its one of those facts with WC3. The locust passive ability just does make the units unselectable and untargetable.
Yes we can explain why the locust ability was programmed with those hard coded properties (for the locusts of the locust swarm ability used by the crypt lord hero) but why the locust passive has these properties and not some other arbitrary passive was a development choice by blizzard that we can not explain.
This is where SC2 data is far more logical than WC3.