I have a question in general about chicken in Japan. I know in some places it's served raw as sashimi, so does that mean that there are no laws to regulate how cooked chicken should be?
I ask because of one situation in particular - I was at a yakitori place with friends and we were eating some (fantastic, delicious) breast-meat chicken. We'd all been eating the pieces in one bite, but a friend cut hers in half and found the middle to be very pink. We were torn on what to do - send it back or assume that's how it's supposed to be.
Is chicken often cooked "medium" here? Or does that seem like a mistake by the kitchen?
I'm no expert, but the only time I had raw chicken was in a very expensive restaurant and the chicken was killed and served the same day.
I highly doubt that a yakitori place would have fresh chicken and I would be worried if I were you.
When yakitori places serve raw or semi-cooked chicken they generally source it from small farms where the birds are raised (and the meat is processed) in hygienic conditions, so that the meat can safely be eaten raw.
Breast meat (sasami) in particular is often prepared the way you describe in good yakitori establishments - rest assured that that's the way they meant to serve it.
Please ignore davidenemy's post - raw, seared or rare chicken is served in very many yakitori places (certainly not just high end), it is absolutely delicious and I have never had any digestive or other problems with it. Only yesterday I went to a yakitori place which did some very good chicken tataki - essentially raw checken seared over some coal - and rare sasami (a certain part of the breast).
While I agree that eating raw or "medium" chicken in Japan is much safer than eating the same in the US or elsewhere, problems do arise. A friend's friend (I know, I know, but it's a true story) suffered a miscarriage because of listeria from eating not-fully-cooked chicken. The friend's friend was Japanese, and this happened in Japan.
Were I sickly or if my immune system were somehow compromised, I would avoid eating raw or semi-cooked chicken in Japan. That being said, I am not, and have no problem with it.
Not sure about the official regulations governing how chicken should be cooked, but in your example, I'm sure it was not a mistake by the kitchen. Chicken that is served raw in the middle or completely raw is very common at yakitori and other jidori chicken restaurants all over Japan. And it's not limited to breast meat, as I've eaten chicken tataki from all parts before.