the unions have done the most damage of all to workers rights with their ever more outlandish demands, meaning that in the end they had to be cut down to size. Hence TGL was a product of union dogma.
a) because it's a nonsensical silly policy, all it does is raise everyone to keep differentials and basically causes inflation so any benefit is wiped out.
b) Probably because it is now politically difficult to reverse it. The Torys are nothing if not pragmatic.
c) see a)