Their big blog post actually explained (IMO) fairly well the logic behind their decision. Patreon works differently from many subscription models which have you

Their big blog post actually explained (IMO) fairly well the logic behind their decision. Patreon works differently from many subscription models which have you on a monthly basis pinned to the date you started. If you start Netflix on the 15th, you'll be billed on the 15th each month. Meanwhile if you start a Patreon pledge on the 15th there's the issue of whether you're charged on the 15th for the first month or on the 1st of the next month. And if the latter, what about pledges that unlock additional rewards, can those people cancel before their first actual payment? Essentially they have a lot of complicated problems to solve, they just picked a solution that introduced a whole lot of new problems for the very people who provide the life blood for the service.