If injury information were covered up completely, and there was no obligation to disclose who is even able to participate in practices, would odds makers be then be tripping all over themselves by any means necessary, including getting this information through an inside informant, player /coach /janitor /caterer /trainer's assistant whomever they can get their mitts on?
Such information is devalued, because it is disclosed for free. Sure there are levels of compliance by the teams and reliability of the injury information, but from a gambling point of view I see nondisclosure as a path to rampant corruption, a path to having internal spies within teams being obligated to gambling interests.