[rinon]

There are often privileges and perks associated with being a patron of a certain creator. These perks aren't pro-rated, thus subscribing late in the month for a day or two might be a viable way to cheat this system. Personally, I don't think this is a huge deal, and, if this is their reasoning, it's a mistake.

[ekimekim]

You could even do it where you pay the full cost immediately, but the difference between that and the prorated cost counts as a credit to the next start-of-month payment. eg. If there's a $100 reward tier and I sign up 60% of the way through the month, I immediately pay $100, then at the start of next month my $100 pledge is discounted by the $60 I overpaid for the previous month, so I only pay $40.

Effectively, it flips the first two months so you pay the full amount immediately but if you stay on for at least two months it all balances out in the end.

EDIT: Ninja'd by sibling comment

[ThrustVectoring]

Charge the pro-rated amount on the first of the first month and the full month's amount on the initial subscription date.