Actually, by the strict dictionary definition, they are:
From Merriam-Webster's Dictionary, with relevant sections in bold -
1scab

Pronunciation: \ˈskab\ Function:
noun Etymology:Middle English, of Scandinavian origin; akin to Old Swedish
skabbr scab; akin to Old English
sceabb scab, Latin
scabere to scratch — more at
shaveDate:13th century1
: scabies of domestic animals2
: a crust of hardened blood and serum over a wound3 a
: a contemptible person b
(1): a worker who refuses to join a labor union (2)
: a union member who refuses to strike or returns to work before a strike has ended (3)
: a worker who accepts employment or replaces a union worker during a strike
(4): one who works for less than union wages or on nonunion terms4
: any of various bacterial or fungus diseases of plants characterized by crustaceous spots;
also : one of the spots
Pilots are the only labor group that think the term "scab" has such a tight definition. I come from a long line of union members on both sides of my family, but they were all blue-collar workers. To them, "scab" can mean pretty much any person that undercuts union wages or work rules, works for a non-union company, doesn't join the union at a union company, etc... My grandfathers and great-grandfathers would think it absurd for a union worker to defend someone that does the same job as them for a third the pay.