the simple fact which i have known about the cap is that it is a symbol of
slavery(ghulami). As u know that mughals have ruled india for hunderds of years.
the deprived indians everything which makes a man feel proud.
These include turban, sword, horse etc. no indian was allowed to wear turban and keep a sword and a horse . this thing can be just confirmed from anyone who has a bit knowledge of history.
besides being compulsory form sikhs in their day to day life ,u will see that
even in hindus the groom has to wear a turban and also for the father of the bride and groom it is necessary.
so apart from other facts u can makeout that sikhs have faced everything to save the diginity of their religion and hence the nation.