Basically the IRS is highly skeptical of the idea that free software development fits the legal definition of a 501(c)(3), and tends to reject such applications [1][2]. That is why Mozilla Foundation cannot use donations for Firefox development, and instead uses them for activism.

So that creates the strange situation where legally it is easier for free software developers to accept donations as a for-profit corporation than as a 501(c)(3) non-profit. It is possible to instead incorporate as a not-for-profit corporation which doesn't have the tax advantages of a 501(c)(3), but does have the advantage of not being beholden to share holders. However, many people react negatively to this assuming that any not-for-profit that isn't a 501(c)(3) is a scam.

[1] https://www.stradley.com/business-vantage-point-blog/irs-con...

[2] https://www.mill.law/blog/more-501c3-rejections-open-source-...