They knew well ahead that Apple would take a 30% cut so I don't understand why some commenters are saying about why this is a "problem."
I'd prefer the money goes to the Document Foundation, who already need to pay to use the app store. It's not a major issue, but it's a downside to the decision in my eyes.
There's a bunch of other related costs associated with commerce on apps. This is why some developers don't "charge" for apps or don't setup their own storefronts. Take credit card charge-backs or fraud disputes. By going through the Apple Storefront, Document Foundation is outsourcing that headache to Apple.
You know what also solves that headache? Offering a cost free download of your software. Like LibreOffice already does for MacOS, which they say is the recommended source for all users.