Some use Ronin to perform vandalism and therefore frequently get banned and buy it again, while Ronin is somewhat expensive for peaceful 2ch users.
So it's reasonable and we will obtain more long-term purchases if you make 1-month Ronin much more expensive and 1-year Ronin much cheaper, like $8 for 1-month and $20 for 1-year. (It's nicer if one can buy 1-month Ronin as a trial and add $12 to switch it to 1-year.)
Under current pricing of $5 for 1-month and $35 for 1-year (even with haloween discount), most users choose 1-month and won't buy it again.
In short: peaceful users can enjoy long-term Ronin lightheartedly at a low price, while trolls cannot.