Gifts do not confer obligation. Copying deprives the original party of nothing. Absolutely nothing about free software requires or even implies any responsibility to “give back”. This idea that anyone making money with free software somehow owes the original authors anything (or “should” donate a portion of their profits) is ridiculous.

If the authors wanted money for their software, they would have sold it instead of giving it away for free as a gift.

By releasing software under free software licenses you are explicitly stating that you do not expect or anticipate payment for it. The licenses (that they freely chose) are clear. Free software, in addition to being free as in speech, is also always free as in beer.

My friend bought me lunch. I used that energy at my job. Do I owe them part of my paycheck?