I thought it was odd that the recently released voice SIM included a cancellation fee since JCI had just removed it from the Talking SIMs. Well, they're back on the Talking SIMs. All new JCI voice SIMs will carry a 3-month minimum requirement and a ¥5,250 early cancellation fee.
This is still much better than the standard 2-year contracts from Japanese carriers, or the 1-year contracts on the Aeon SIMs, but it does leave the impression that JCI is figuring things out as they go along.
Without doubt, JCI does incur costs to Docomo for each new contract, so I would guess this new fee is meant to primarly cover those costs, rather than attempt to "tie" customers into service. If shibari was the goal, why only have a 3-month requirement?
UPDATE: there is an excellent comment below explaining that, as you might imagine, many people took advantage of the low cost of entry and exit to use it as a stepping stone into a contract with a Japanese carrier. Porting in an existing phone number can qualify you for a number of discounts that can either be significant up-front cash or add up to several hundred bucks over the course of a 2-year contract.
This is of course what happened in the first place when many resellers started offering large cash-back incentives. I still think it was odd that 1) the smartphone voice SIM was introduced at first without a cancelation fee, and 2) that they didn't immediately add the same fee to the talking SIMs.