Actually, technically I can’t say they are using wrong English. The eighth edition of the Oxford Advanced Learner’s Dictionary (OALD) actually says that the word is of Indian origin and given that we Indians significantly contribute to the dynamics of the English language, it is really ok to use the word. It rather serves the purpose too, because it means to bring forward to an earlier date or time. For example,
Your flight no XXX to YYY has been preponed to 10.30 am from 12.15 pm.
For someone like me who prefers strict, corseted English, it is of some comfort that the word has its etymology in Latin:
prae + pōnō.
So yes, if you can postpone, I suppose you can prepone too. I mean, if it follows the same morphology, I don’t suppose anyone can blame us Indians for coining a rather handy, and if I may say so, popularly used word, eh?
I am not so sure about its usage around the world, so play it safe and use it only in India, ok? Ok