I hate to get in on this discussion, but I feel I must offer a balancing counter point. All Indians do not speak and write English at the same level. There a hundreds of school systems in India. Some teach English from kindergaten and others from Grade 5 or later.
There are school systems within India (relatively large ones) that teach kids the works of Shakespeare before they are teenagers. Several of the Indian applicants to business schools come from schooling backgrounds I have described.
Stereotyping in this situation may be easy... but unfortunate. India is an extremely diverse land in which everyone has a voice. Some of these voices use articles more infrequently than others.
Also, have you considered the possibility that the Indians who have good command over the language may not need your essay-editing services?
I don't mean to bag on you because I know you mean well. I just wanted to set the record straight.