Oh, message board legal debates; how would we occupy our days without you? I love you most of all, "If I just did X, they couldn't charge me with Y" argument!
18 USC § 2 - PRINCIPALS
(a) Whoever commits an offense against the United States
or aids, abets, counsels, commands, induces or procures
its commission, is punishable as a principal.
(b) Whoever willfully causes an act to be done which if
directly performed by him or another would be an offense
against the United States, is punishable as a principal.
Net net: I'd burn that stupid "DRUGS R HERE" sign.