No, it's not illegal, so long as (1) the restaurant has the proper and appropriate liquor license; and (2) the customers are legal age to drink. Of course, if the restaurant is a franchise or owned by some other, larger company, it might be against the rules or the franchisor or owner--that is, the franchisor or owner may make it a policy to not offer a complementary glass of wine. But the law would allow it, so long as there is a liquor license and legal age customers.