Service Charge in Restaurants

My friend and I went to this restaurant in Chetpet - Kailash Parbat. My friend insisted he’ll pay for the bill. I wanted to make sure we are billed for the items we ordered when I noticed that we were charged 10% service tax. My friend enquired (in Hindi) regarding those charges to which without hesitation they said it’s optional and agreed to take it off the bill. Does this happen in all restaurants you visit? Is this even legal?