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.
By statute, an organization can only exist as a non-profit in the US if it is "organized and operated exclusively for exempt purposes"[0], and non-profit hospitals specifically must operate for charitable purposes.
It may not be wise to assume non-profits exist within the confines of the law that authorizes their continued existence, but I don't see how it's fallacious.
[0]: https://www.irs.gov/charities-non-profits/charitable-organiz...
The concept of having a 'non-profit' tax category was explicitly to allow for (and even encourage the creation of) altruistic endeavors. That's why we have them. I do believe the original motivations for the non-profit category were sincere in this regard.
However once such a tax category was created, there was really nothing to stop the sociopathic MBA class from using them as just another optimization tool in their tax-minimization arsenal. Another example is non-profit schools where the property is owned by the founders and they charge hugely non-market rents to personally extract revenue from the non-profit.
So in current days we have both genuine altruistic endeavors (they still exist) and the predatory ones abusing the system.
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.