Sushi rice seasoning does indeed contain a fair bit of sugar and this explains the syns. With store-bought sushi I wouldn't be in the least surprised if they used more sugar in there. Also you quite often get mayonnaise in the maki rolls. Seven syns for a pack from the shop sounds about right but as ScarlettStar says you can't allow for two freefoods in there.
Wasabi is synned but goodness only knows why. And although the book says all dried seaweed is free, the syns online says some brands of nori sheets are synned, despite them being 100% dried seaweed. This has never been adequately explained to me whenever I've asked.
If you make sushi yourself you can make it free. It is so incredibly healthy I wouldn't worry too much about it. Look at the ingredients and see how high the sugar and mayo content is, if it isn't then really don't worry about it.