The 6 month notice rule was basically designed back in the day to help out AI & Indian Airlines and prevent mass exodus from the state sponsored companies.
The state sponsored airlines paid for all expense of crew training and that is why they could ask for such thing..
It got carried over to private airlines as well and that is where it is wrong.
The private airlines either hire type-rated guys or ask people to pay for their type rating, so they have no moral grounds for asking for a 6 month notice period.
It definitely not constitutional and will not hold well at all in case there is a writ petition in Supreme Court.
The problem is who will bell the cat.....