Your line of reasoning seems to be along the lines of "anything not forbidden is compulsory" which is not a good place. Having the choice to donate but not being required to is good.
And yes, I think that if OpenAI is choosing to include an entire copy of LibreOffice in their product, it is definitely the right thing to do to sling them a donation. Note that the moral thing to do is not always the required thing to do, or even the legal thing to do.