Why is it considered unprofessional for the security agency to refuse to protect corrupt politicians in India?

You are mixing up things.Security agencies have a job cut out for them and the antecedents /character of the person involved is not their concern as there are other agencies to look after that part.Take example of a notorious gangster and a court may ask security agencies to protect him from outrage /anger of the general public or a rival group.Unless it is proved in the court that a politician is corrupt,no action can be taken against him.The law of the land is clear and we are a democratic country,where Rights of every person are protected.On all other aspects,I agree with you.