Yes, when people are posting your trailer all over the interwebs, that is a copyright violation, and it probably throws a spanner in the works of your carefully planned schedule for drip feeding information to risk-averse press outlets desperate for content.
But remember how you're also spending loads of money on social networking and stooges to generate hype and maybe even "go viral"? Well this is how hype is generated and virals are gone. This is the public doing the job for you, organically, in a way that others will listen to rather than distrust. This is you on the cusp of explosive, exponential growth, and the last thing it needs is you stamping on the embers.
And, oh yeah, the best trailer in the world is worth absolutely nothing if I can't bloody see it before I forget it even exists.