http://www.reuters.com/article/2013/04/12/us-trade-asiapacif...
(edit): Here's a whole article about US TPP pressure against kei cars:
http://www.japantimes.co.jp/news/2012/01/27/business/lower-t...
"When American automakers expressed their opposition against Japan
joining the Trans-Pacific Partnership negotiations due to its closed
market, the U.S. makers also pressed Tokyo to end special tax
treatment for the country’s unique “kei” super-minivehicle segment."
"The outside pressure to end the low taxation for one of Japan’s most
popular car segments has jolted the industry, which has protected the
smaller end of the domestic car market for half a century."
"Some industry observers point out that reviewing the tax rules for
the super-mini segment would hit Japanese automakers hard. But others
say a review would be a good thing because kei cars have received
favorable treatment for too long."
Reading those two articles, I think it's credible that US negotiators got a concession from Abe on kei taxes, and we're seeing the result of that. As I understand, this kind of deal would be made in secret (like many aspects of TPP), so we wouldn't be aware of it at this point.