When judges move from the Department of Justice to a court, the rules of judicial ethics require them to recuse (disqualify) themself in any case they worked on as a government lawyer. Rehnquist was recused from many cases during his first term on the Court, for the same reason. By next term, this won't be an issue.
There are many things to criticize Kagan for, but this isn't one of them; she is following the law.
We should be happy that she is forced to recuse herself from many cases this term. We knew we were going to get a leftist, but at least she can’t do as much harm as someone who wouldn’t have to recuse themselves. In a close case, it could be the difference between winning it and losing it even if Kennedy votes with the socialists.