First, if it's being organized by the President of the United States it is by definition not a "grassroots movement." Idiots.
"Grassroots" connotes a spontaneous, local gathering of like-minded people who are usually every-day Joes rather than typical political activists. It is a bottom-up formation--"We the people believe"--rather than to the all too common top-down, "Let me tell you what you believe," favored by politicians.
So when Obama sends an email to all his Obamatons that "On June 6th, in thousands of homes across the country, we'll gather to launch our grassroots campaign for health care," he has completely missed the point. He (and, never forget, his minders) want the media to report that this is a grassroots movement because that type of thing sounds good on TV.
So he's either ignorant or being... less than truthful.
Or maybe it's the President's classic "I'll invert the actual meaning of words."