This seems relevant:

Five: US tax law prohibits public charities from getting too much support from big donors.

Under US tax law, a 501(c)(3) public charity must maintain a certain percentage of "public support". As with most tax rules, this one is complicated. If, over a four-year period, any one individual donates more than 2% of the organization's total support, anything over 2% does not count as "public support". If a single donor supported a charity, its public support percentage would be only 2%. If two donors supported a charity, its public support percentage would be at most 4%. Public charities must maintain a public support percentage of at least 10% and preferably 33.3%. Small donations - donations of less than 2% of our total support over a four-year period - count entirely as public support. Small donations permit us to accept more donations from our major supporters without sending our percentage of public support into the critical zone. Currently, the Singularity Institute is running short on public support - so please don't think that small donations don't matter!