Agreed. Mucking about with affiliate programs and payment plans is not really worth it - they have a very good product at an extremely affordable price, comparative to the market. $140 is pretty reasonable for a commercial forum package, there's no need to adulterate it.
Protip: If you want the software in instalments, just save $70 a month for two months, or $35 for four months. Sure, you don't get it instantly, but you've got no contract or no slightly dodgy affiliate scheme to deal with either.