Users buying LibreOffice on the Mac App Store are not going to be doing so because they care about free software – if they did they probably wouldn't be on a Mac and would probably download from official channels, perhaps donating directly if they want to contribute so that Apple don't take 30%.
This means that those users buying it are users who would otherwise buy/use iWork, Microsoft Office, or Google Docs, and I'm pretty sure they're going to be in for a bit of a shock. LibreOffice is usable, but it's not the same in terms of usability.
So, they're about to get a bunch of users who stumble onto their software, pay for it, have a bad experience, and then expect and are essentially entitled to customer support. Customer support through the App Store is hard at the best of times, so good luck. I somehow doubt the Document Foundation has factored this in to their pricing, or that they even have a customer support team.