http://www.oxforddictionaries.com/words/apostrophe#apostrophes_showing_possession
From the official Oxford English Dictionary. These people decide how the English language works. Also used on the UK TV show Countdown. If you have an objection, take it up with them.
Boom.
Actually, speakers and writers of English are the ones who decide how the English language works. Dictionary writers are not authorities, and their job is actually to represent the language as used by the speakers in the most accurate and correct way possible. Languages evolve as those who speak and write them change how they speak and write them, and dictionaries are to reflect this reality, not force it.
That's why the acceptance of the odd syntax of allowing singular possessive to drop the s after the apostrophe is often referred to as "recent", because dictionaries are noticing its usage. Oxford, in that link, says exception is occasionally made especially for organization names, and that it is best to check with the organization on how they spell it - because, again, Oxford is just trying to reflect the reality of an evolving language, and not act as an authority. And since s's is pronounced as such (Thomas's cheese as "Thomasis cheese", not just "Thomas cheese"), Oxford is quite clear in that link that you should indeed spell it correctly: Thomas's cheese.
Can I get a boom?
