Now issue of Aadhar Card is governed by Aadhar Card Act. You may go through the act by clicking on the following link:
Now coming to your question, while approachin an Employer for employment, you should give such employer your proof of identity. Aadhar is one document the Employer is asking. The Employer might have asked for alternate document also. You may verify with the employer.
Legality in asking for Aadhar Card by the Employer is perfect and there is nothing illegal in that. If you do not agree with this, what alternative you have?? Filing of a case!! Whether it will solve your employment problem?