I suspect that SIAI is in a different position from most charities.

I don't know what percentage of charities are low on public support, but I suspect that is not a serious issue for most donors, as most donors couldn't provide more than 2% of a charity's total income, even with a third of their total charity budget.

Most charities have a practice of sending endless streams of junk mail, and so for most charities a gift of a few dollars is actually a losing proposition in the long term, since you sent the signal you would be receptive to future donation requests but don't actually send more money. The SIAI's strategy (and costs for emailing) are different from most charities, suggesting that different advice makes sense for them.