The reason is if these conpanies paid for t.v. ads (like they do most of their drugs) themselves they would have to list their potential side effects in the ad... like every pharma ad on t.v.
They get around it because when the Fed, a municipality, or any 3rd party is advertising a medicine they AREN'T required to list side effects.
Why pay for a 30 second time slot on nbc when you can just have the federal government force the citizens to take it.