Yes, legalities...
"Translation: Mr. Buffett received special permission from the S.E.C. to keep secret his investment in I.B.M. — and possibly keep secret stakes in other companies that he is building positions in that we have yet to learn about.
Mr. Buffett’s special treatment from the S.E.C. is not new — he has long taken advantage of an obscure rule to avoid disclosing his bets to the public before he is good and ready."
http://dealbook.nytimes.com/2011/11/14/one-secret-buffett-gets-to-keep/