It comes from certain specific ancient theologies which adopt philosophical positions of endorsing anonymous giving.

In christian theology, for example, there is the principle "do not let your left hand know what your right hand is doing". Back in those days some almsgivers would carry a bell and ring it as they gave out money to the poor, calling attention to themselves. It's possibly the bell ringing practice originated in wanting to let the poor know that alms were available, but christian texts clearly indicate that it had become a way of calling attention to oneself for their charity works. To this end, some persons donated in order to receive admiration from others, not out of compassion for the less fortunate. It was thus mentioned "they have already received their reward". This was contrasted to anonymous giving and a future reward in heaven as an alternative. But note still the person is doing it in expectation of receiving a reward, and not out of a sense of compassion.

Unfortunately, in the modern times, these principles are not normally brought up by others that share one's religion, but rather as a debate tactic to make others look bad and put down those who are trying to make a difference, regardless of any faith issues.

This principle is often brought up, as we see here, in order to pass judgment against those who give charity. Those who pass this judgment sit in self-righteous judgment of others rather than "minding their own business" as these judeo-christian writings also implore. Thus those making such pronouncements are picking and choosing that which makes them happy. The judgers judge because it makes them feel good about themselves, not because they are members of the same church as Larry Page (who is actually atheist, according to various sources) or others who are merely concerned with them having the right motivation in alignment with their communal belief system. The judgers have received their reward - a smug feeling of superiority over others who work to help the less fortunate.