As much I know locust is a passive abilty that hasn't got any value. Also its a basic ability in wc3, every map has it. Other abilities can be remade and/or removed ( Because they have values ).
With unhiding those "Build - *Race" abilities you would be able to change build command, as well as the data of it. wc3 engine needs to have those 4 basic abilities for each worker with that values, else it will crash with fatal error.
( Syntax check with build values - Yes the engine was not very good made ).
Locust is also passive with nonchangeable values, build is active with many values to build ( I think start hp, hp for each second it builds, etc. )
Btw, I may be wrong with this but I think it isn't.