Depends on whether you view your donation as a good will gesture/reimbursement of sorts for services rendered – or if you consider in an investment with more clearly defined objectives at the end of the fundraising campaign.
I suspect it's just basic human behaviour. If other people are doing it, then it's somewhat normalised.
Also personally, it seems to me that if you have a goal, then that's a minimum. If you don't reach it, the thing doesn't happen. A website never reaching its funding goals is one that seems more likely to shut down, and what's the point of supporting something that could shut down tomorrow because it can't pay it's way. I know that's self reinforcing but there it is.
