Assuming that non-profits are altruistic seems fallacious. Granted, I don’t know why they are assumed to be by some; it’s just presented as such because it seems obvious, no arguments need to be given.
It’s clearly fallacious to assume that non-profit is altruistic just because, I don’t know, for-profit is assumed as a premise to be about egotistical money hoarding.
I think the assumption about non-profits being altruistic is a reasonable one, because what would otherwise be the justification for giving them tax breaks?
If the reality is different, then maybe there shouldn't be non-profits anymore. In the UK for example, there are no non-profits, there are only charities. And clearly, the expectation of altruism is explicit here.