There does not need to be a written contractual relationship to invoke tortious interference. The followers clearly intended to support Owen financially, not Patreon itself, so there is a financial relationship between them.

But given that Patreon was clearly and obviously a man-in-the-middle of the relationship and had policies which could be violated and which would terminate the relationship, and all parties knew this up front, then it is also clear to me that Patreon can freely choose to stop being a man-in-the-middle, and as such I don't think the tortious interference claim makes sense in this case.